doc: Update URL in pointers to F.16 C++ Core Guidelines (#1232)

The anchor ID in the target is now lower-case.

Author: maflcko

include/gsl/pointers

@@ -51,7 +51,7 @@ namespace details
     // Resolves to the more efficient of `const T` or `const T&`, in the context of returning a const-qualified value
     // of type T.
     //
-    // Copied from cppfront's implementation of the CppCoreGuidelines F.16 (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-in)
+    // Copied from cppfront's implementation of the CppCoreGuidelines F.16 (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#rf-in)
     template<typename T>
     using value_or_reference_return_t = std::conditional_t<
                                             sizeof(T) <= 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,