fix of sample implementation

Author: rotarymars

reference/algorithm/ranges_mismatch.md

@@ -196,7 +196,7 @@ struct mismatch_impl {
   template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
     requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
   constexpr mismatch_result<I1, I2> operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const {
-    for ( ; first1 != last1 && first != last2; ++first1, ++first2)
+    for ( ; first1 != last1 && first2 != last2; ++first1, ++first2)
       if (!bool(invoke(pred, invoke(proj1, *first1), invoke(proj2, *first2))))
         return {first1, first2};
     return {first1, first2};