Fix CONTRIBUTING.md link (#1752)

* Fix CONTRIBUTING.md link

* Update CONTRIBUTING.md

Co-authored-by: Matt Moore <[email protected]>

Co-authored-by: Matt Moore <[email protected]>

Author: shuheiktgw

CONTRIBUTING.md

@@ -1,5 +1,5 @@
 # Contribution guidelines
 
 So you want to hack on Knative `pkg`? Yay! Please refer to Knative's overall
-[contribution guidelines](https://github.com/knative/docs/blob/master/community/CONTRIBUTING.md)
-to find out how you can help.
+[contribution guidelines](https://knative.dev/contributing/) to find out how you
+can help.