fix: correct the contributing pr template link

Author: RomainOliva

.github/PULL_REQUEST_TEMPLATE.md

@@ -29,7 +29,7 @@ Example: The exact commands you ran and their output, screenshots / videos if th
 
 Please add a `x` inside each checkbox:
 
-- [ ] I have read the [contribution guidelines](../CONTRIBUTING.md).
+- [ ] I have read the [contribution guidelines](https://github.com/netlify/netlify-cms/blob/master/CONTRIBUTING.md).
 - [ ] Code is formatted via running `yarn format`.
 - [ ] Tests are passing via running `yarn test`.
 - [ ] The status checks are successful (continuous integration). Those can be seen below.