YARN-9945. Fix javadoc in FederationProxyProviderUtil in branch-2

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-common/src/main/java/org/apache/hadoop/yarn/server/federation/failover/FederationProxyProviderUtil.java

@@ -28,6 +28,7 @@ import org.apache.hadoop.security.UserGroupInformation;
 import org.apache.hadoop.security.token.Token;
 import org.apache.hadoop.security.token.TokenIdentifier;
 import org.apache.hadoop.yarn.client.AMRMClientUtils;
+import org.apache.hadoop.yarn.client.ClientRMProxy;
 import org.apache.hadoop.yarn.client.RMFailoverProxyProvider;
 import org.apache.hadoop.yarn.conf.HAUtil;
 import org.apache.hadoop.yarn.conf.YarnConfiguration;