Fixed instance register mistake that not return instance id but application

apm-collector/apm-collector-agentregister/src/main/java/org/skywalking/apm/collector/agentregister/instance/InstanceIDService.java

@@ -33,7 +33,7 @@ public class InstanceIDService {
                 logger.error(e.getMessage(), e);
             }
         }
-        return applicationId;
+        return instanceId;
     }
 
     public void recover(int instanceId, int applicationId, long registerTime, String osInfo) {