fix javadoc.

Author: hfutatzhanghb

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/router/RouterRpcClient.java

@@ -282,7 +282,7 @@ public ActiveNamenodeResolver getNamenodeResolver() {
 
   /**
    * Get the executor service used by invoking concurrent calls.
-   * @return
+   * @return the executor service.
    */
   @VisibleForTesting
   public ThreadPoolExecutor getExecutorService() {