[mutex.syn] Add missing ',' for consistency

Author: frederick-vs-ja

source/threads.tex

@@ -6456,7 +6456,7 @@
   class mutex;
   // \ref{thread.mutex.recursive}, class \tcode{recursive_mutex}
   class recursive_mutex;
-  // \ref{thread.timedmutex.class} class \tcode{timed_mutex}
+  // \ref{thread.timedmutex.class}, class \tcode{timed_mutex}
   class timed_mutex;
   // \ref{thread.timedmutex.recursive}, class \tcode{recursive_timed_mutex}
   class recursive_timed_mutex;