[string.view.cons] Change "R" to "remove_reference_t<R>" in constraint.

The reference removal was omitted from P1989R2 accidentally. Without it, the entire constraint item would not apply when R is a reference type, even though it was clearly intended to cover that case.

Author: tkoeppe

source/strings.tex

@@ -4309,7 +4309,8 @@
 \tcode{d.operator ::std::basic_string_view<charT, traits>()}
 is not a valid expression, and
 \item
-if the \grammarterm{qualified-id} \tcode{R::traits_type} is valid and denotes a type,
+if the \grammarterm{qualified-id} \tcode{remove_reference_t<R>::traits_type}
+is valid and denotes a type,
 \tcode{is_same_v<remove_reference_t<R>::traits_type, traits>} is \tcode{true}.
 \end{itemize}