[istreambuf.iterator.general] Add \ref for proxy (#5669)

Author: hewillk

source/iterators.tex

@@ -6225,6 +6225,7 @@
     using streambuf_type    = basic_streambuf<charT,traits>;
     using istream_type      = basic_istream<charT,traits>;
 
+    // \ref{istreambuf.iterator.proxy}, class \tcode{istreambuf_iterator::\exposid{proxy}}
     class @\placeholder{proxy}@;                          // \expos
 
     constexpr istreambuf_iterator() noexcept;