Update PR template link to master CONTRIBUTING.md (#423)

Currently `CONTRIBUTING.md` links to `/CONTRIBUTING.md`, it's fine for markdown rendering but in the PR, it links to https://github.com/contributing.md instead

Co-authored-by: Jonathon Belotti <[email protected]>

Author: ducthienbui97

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,7 @@
 
 Please check if your PR fulfills the following requirements:
 
-- [ ] Does not include precompiled binaries, eg. `.par` files. See [CONTRIBUTING.md](/CONTRIBUTING.md) for info
+- [ ] Does not include precompiled binaries, eg. `.par` files. See [CONTRIBUTING.md](https://github.com/bazelbuild/rules_python/blob/master/CONTRIBUTING.md) for info
 - [ ] Tests for the changes have been added (for bug fixes / features)
 - [ ] Docs have been added / updated (for bug fixes / features)