Fix no instance of overloaded function stan::math::Holder::coeffRef

```c++
R/library/StanHeaders/include/stan/math/prim/fun/scalar_seq_view.hpp(32): error: no instance of overloaded function "stan::math::Holder<ArgType, Ptrs...>::coeffRef [with ArgType=Eigen::ArrayWrapper<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0, Eigen::Stride<0, 0>>, std::remove_reference_t<std::vector<double, std::allocator<double>>>>>, Ptrs=<std::remove_reference_t<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0,
          Eigen::Stride<0, 0>>, std::remove_reference_t<std::vector<double, std::allocator<double>>>>>>]" matches the argument list and object (the object has cv-qualifiers that prevent a match)
            argument types are: (size_t)
            object type is: const stan::ref_type_t<stan::math::Holder<Eigen::ArrayWrapper<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0, Eigen::Stride<0, 0>>, std::remove_reference_t<std::vector<double, std::allocator<double>>>>>, std::remove_reference_t<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0, Eigen::Stride<0, 0>>, std::remove_reference_t<std::vector<double, std::allocator<double>>>>>> &&>
    inline auto operator[](size_t i) const { return c_.coeffRef(i); }
                                                       ^
R/library/RcppEigen/include/Eigen/src/Core/ArrayBase.h(70): note: this candidate was rejected because mismatch in count of arguments
      using Base::coeffRef;
                  ^
R/library/RcppEigen/include/Eigen/src/Core/ArrayBase.h(70): note: this candidate was rejected because the selector is mismatched
      using Base::coeffRef;
                  ^
          detected during:
            instantiation of "auto stan::scalar_seq_view<C, stan::require_eigen_vector_t<C>>::operator[](size_t={unsigned long}) const [with C=stan::ref_type_t<stan::math::Holder<Eigen::ArrayWrapper<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0, Eigen::Stride<0, 0>>, std::remove_reference_t<std::vector<double, std::allocator<double>>>>>, std::remove_reference_t<stan::math::Holder<Eigen::Map<Eigen::Matrix<double, -1, 1, 0, -1, 1>, 0, Eigen::Stride<0, 0>>,
                      std::remove_reference_t<std::vector<double, std::allocator<double>>>>>> &&>]" at line 87 of "R/library/StanHeaders/include/stan/math/prim/prob/gamma_lpdf.hpp"
            instantiation of "stan::return_type_t<T_y, T_shape, T_inv_scale> stan::math::gamma_lpdf<propto,T_y,T_shape,T_inv_scale,<unnamed>>(const T_y &, const T_shape &, const T_inv_scale &) [with propto=false, T_y=std::vector<stan::math::var_value<double, void>, std::allocator<stan::math::var_value<double, void>>>, T_shape=double, T_inv_scale=double, <unnamed>=(void *)nullptr]" at line 1547 of "stan_files/polr.hpp"
            instantiation of "stan::scalar_type_t<VecR> model_polr_namespace::model_polr::log_prob_impl<propto__,jacobian__,VecR,VecI,<unnamed>,<unnamed>>(VecR &, VecI &, std::ostream *) const [with propto__=false, jacobian__=false, VecR=Eigen::Matrix<stan::math::var, -1, 1, 0, -1, 1>, VecI=Eigen::Matrix<int, -1, 1, 0, -1, 1>, <unnamed>=(void *)nullptr, <unnamed>=(void *)nullptr]" at line 2116 of "stan_files/polr.hpp"
            instantiation of "T_ model_polr_namespace::model_polr::log_prob<propto__,jacobian__,T_>(Eigen::Matrix<T_, -1, 1, 0, -1, 1> &, std::ostream *) const [with propto__=false, jacobian__=false, T_=stan::math::var]" at line 96 of "R/library/StanHeaders/include/src/stan/model/model_base_crtp.hpp"
            instantiation of "stan::math::var stan::model::model_base_crtp<M>::log_prob(Eigen::Matrix<stan::math::var, -1, 1, 0, -1, 1> &, std::ostream *) const [with M=model_polr_namespace::model_polr]"
```

Author: hsbadr

stan/math/prim/fun/scalar_seq_view.hpp

@@ -29,7 +29,7 @@ class scalar_seq_view<C, require_eigen_vector_t<C>> {
    * @param i index
    * @return the element at the specified position in the container
    */
-  inline auto operator[](size_t i) const { return c_.coeffRef(i); }
+  inline auto operator[](size_t i) const { return c_.coeff(i); }
 
   inline auto size() const noexcept { return c_.size(); }