YARN-8610. Fixed initiate upgrade error message.

Contributed by Chandni Singh

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-applications/hadoop-yarn-services/hadoop-yarn-services-core/src/main/java/org/apache/hadoop/yarn/service/client/ServiceClient.java

@@ -257,7 +257,7 @@ public class ServiceClient extends AppAdminClient implements SliderExitCodes,
     if (!liveService.getState().equals(ServiceState.STABLE)) {
       String message = service.getName() + " is at " +
           liveService.getState()
-          + " state, upgrade can not be invoked when service is STABLE.";
+          + " state and upgrade can only be initiated when service is STABLE.";
       LOG.error(message);
       throw new YarnException(message);
     }