[ostream.inserters] Add missing \tcode to index entry

Author: tkoeppe

source/iostreams.tex

@@ -6425,7 +6425,7 @@
 return *this << s;
 \end{codeblock}
 where \tcode{s} is an
-\impldef{NTCTS in basic_ostream<charT, traits>\& operator<<(nullptr_t)}
+\impldef{NTCTS in \tcode{basic_ostream<charT, traits>\& op\-er\-ator<<(nullptr_t)}}
 NTCTS~(\ref{defns.ntcts}).
 \end{itemdescr}