[functional.syn] Add missing \tcode (#5806)

Author: hewillk

source/utilities.tex

@@ -9923,7 +9923,7 @@
            class BinaryPredicate = equal_to<>>
     class boyer_moore_horspool_searcher;
 
-  // \ref{unord.hash}, class template hash
+  // \ref{unord.hash}, class template \tcode{hash}
   template<class T>
     struct hash;                                                                    // freestanding