[skip ci]Fix golint for isOnline (#12664)

Signed-off-by: Nxige-16 <xi.ge@zilliz.com>

internal/querycoord/cluster.go

@@ -726,7 +726,7 @@ func (c *queryNodeCluster) isOnline(nodeID int64) (bool, error) {
 		return node.isOnline(), nil
 	}
 
-	return false, fmt.Errorf("IsOnline: query node %d not exist", nodeID)
+	return false, fmt.Errorf("isOnline: query node %d not exist", nodeID)
 }
 
 //func (c *queryNodeCluster) printMeta() {