Merge pull request #2632 from hsbadr/fix/scalar_seq_view

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

Author: SteveBronder

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(); }