address the comment

Signed-off-by: Ryan Leung <rleungx@gmail.com>

client/pd_service_discovery.go

@@ -576,8 +576,7 @@ func (c *pdServiceDiscovery) updateServiceModeLoop() {
 	defer cancel()
 	ticker := time.NewTicker(serviceModeUpdateInterval)
 	failpoint.Inject("fastUpdateServiceMode", func() {
-		ticker.Stop()
-		ticker = time.NewTicker(10 * time.Millisecond)
+		ticker.Reset(10 * time.Millisecond)
 	})
 	defer ticker.Stop()