[thread.jthread.class] close namespace in synopsis

Author: sebastianrath

source/threads.tex

@@ -1523,6 +1523,7 @@
   private:
     stop_source ssource;        // \expos
   };
+}
 \end{codeblock}
 
 \rSec3[thread.jthread.cons]{Constructors, move, and assignment}