[rcu.syn] Add missing ',' in comment (#7301)

Author: Andreas-Krug

source/threads.tex

@@ -12194,7 +12194,7 @@
   // \ref{saferecl.rcu.domain}, class \tcode{rcu_domain}
   class rcu_domain;
 
-  // \ref{saferecl.rcu.domain.func} non-member functions
+  // \ref{saferecl.rcu.domain.func}, non-member functions
   rcu_domain& rcu_default_domain() noexcept;
   void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept;
   void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept;