[string.access] Use (const_)reference for consistency

with other sequence containers

Author: frederick-vs-ja

source/strings.tex

@@ -3135,8 +3135,8 @@
 
 \indexlibrarymember{front}{basic_string}%
 \begin{itemdecl}
-constexpr const charT& front() const;
-constexpr charT& front();
+constexpr const_reference front() const;
+constexpr reference front();
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3151,8 +3151,8 @@
 
 \indexlibrarymember{back}{basic_string}%
 \begin{itemdecl}
-constexpr const charT& back() const;
-constexpr charT& back();
+constexpr const_reference back() const;
+constexpr reference back();
 \end{itemdecl}
 
 \begin{itemdescr}