[format.arg] Add std:: for forward

Author: hewillk

source/utilities.tex

@@ -17236,7 +17236,7 @@
 \begin{itemdescr}
 \pnum
 \effects
-Equivalent to: \tcode{return visit(forward<Visitor>(vis), arg.value);}
+Equivalent to: \tcode{return visit(std::forward<Visitor>(vis), arg.value);}
 \end{itemdescr}
 
 \rSec3[format.arg.store]{Class template \exposid{format-arg-store}}