Add the missing comment (#10169)

Motivation:

The comment of SingleThreadEventExecutor#run() is missing

Modification:

Just fill it in.

Result:

Method comment is not blank now.

common/src/main/java/io/netty/util/concurrent/SingleThreadEventExecutor.java

@@ -537,7 +537,7 @@ public abstract class SingleThreadEventExecutor extends AbstractScheduledEventEx
     }
 
     /**
-     *
+     * Run the tasks in the {@link #taskQueue}
      */
     protected abstract void run();