docs: update badge

Signed-off-by: Bo-Yi Wu <[email protected]>

Author: appleboy

README.md

@@ -1,6 +1,6 @@
 # NSQ
 
-[![Run Tests](https://github.com/golang-queue/nsq/actions/workflows/go.yml/badge.svg?branch=master)](https://github.com/golang-queue/nsq/actions/workflows/go.yml)
+[![Run Tests](https://github.com/golang-queue/nsq/actions/workflows/go.yml/badge.svg?branch=main)](https://github.com/golang-queue/nsq/actions/workflows/go.yml)
 [![codecov](https://codecov.io/gh/golang-queue/nsq/branch/master/graph/badge.svg?token=V8A1WA0P5E)](https://codecov.io/gh/golang-queue/nsq)
 
 NSQ as backend with [Queue package](https://github.com/golang-queue/queue) (A realtime distributed messaging platform)