fix: CONTRIBUTING.md url error. (apache#1303)

Author: stulzq

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,5 @@
 <!--  Thanks for sending a pull request!
-Read https://github.com/apache/dubbo-go/blob/master/contributing.md before commit pull request.
+Read https://github.com/apache/dubbo-go/blob/master/CONTRIBUTING.md before commit pull request.
 -->
 
 **What this PR does**: