[format.syn] Add missing reference to [format.context] in header synopsis (#3954)

Author: gamecentric

source/utilities.tex

@@ -19125,6 +19125,7 @@
   using format_parse_context = basic_format_parse_context<char>;
   using wformat_parse_context = basic_format_parse_context<wchar_t>;
 
+  // \ref{format.context}, class template \tcode{basic_format_context}
   template<class Out, class charT> class basic_format_context;
   using format_context = basic_format_context<@\unspec@, char>;
   using wformat_context = basic_format_context<@\unspec@, wchar_t>;