[thread.stoptoken.syn] "class template", not just "class"; fix spacing (#5799)

Author: hewillk

source/threads.tex

@@ -498,9 +498,9 @@
   };
   inline constexpr nostopstate_t nostopstate{};
 
-  // \ref{stopcallback}, class \tcode{stop_callback}
+  // \ref{stopcallback}, class template \tcode{stop_callback}
   template<class Callback>
-  class stop_callback;
+    class stop_callback;
 }
 \end{codeblock}