Fix a broken link

Author: bbatsov

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 Before submitting a PR make sure the following things have been done:
 
-- [ ] The commits are consistent with our [contribution guidelines](CONTRIBUTING.md)
+- [ ] The commits are consistent with our [contribution guidelines](../blob/master/.github/CONTRIBUTING.md)
 - [ ] You've added tests to cover your change(s)
 - [ ] All tests are passing
 - [ ] The new code is not generating reflection warnings