[string.cons] Add last begin->end fix-up for P3016R6

Author: AlisdairM

source/strings.tex

@@ -2795,7 +2795,7 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-return *this = basic_string_view<charT, traits>(il.begin(), il.size());
+return *this = basic_string_view<charT, traits>(il.data(), il.size());
 \end{codeblock}
 \end{itemdescr}