docs(Algebra/MonoidAlgebra/Basic): fix order of arguments in docs (#7202)

The arguments to `AddMonoidAlgebra` were reversed with respect to normal use in the module docs.  They also used `α` instead of `R`.  For reference, these are the actual definitions:

```lean
-- Data/MvPolynomial/Basic
def MvPolynomial (σ : Type*) (R : Type*) [CommSemiring R] :=
  AddMonoidAlgebra R (σ →₀ ℕ)

-- Data/Polynomial/Basic
structure Polynomial (R : Type*) [Semiring R] where ofFinsupp ::
  toFinsupp : AddMonoidAlgebra R ℕ
```

Author: adomani

Mathlib/Algebra/MonoidAlgebra/Basic.lean

@@ -30,8 +30,8 @@ in the same way, and then define the convolution product on these.
 
 When the domain is additive, this is used to define polynomials:
 ```
-Polynomial α := AddMonoidAlgebra ℕ α
-MvPolynomial σ α := AddMonoidAlgebra (σ →₀ ℕ) α
+Polynomial R := AddMonoidAlgebra R ℕ
+MvPolynomial σ α := AddMonoidAlgebra R (σ →₀ ℕ)
 ```
 
 When the domain is multiplicative, e.g. a group, this will be used to define the group ring.