doc: fix the PR template CONTRIBUTING.md link

Using ./CONTRIBUTING.md in the PR template resolves to
https://github.com/nodejs/citgm/compare/CONTRIBUTING.md. Use the full path
instead (as is done in Node.js core).

Note that the CONTRIBUTING.md file hasn't yet been added (the PR to add it is
nodejs#314).

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,5 +9,5 @@ the requirements below.
 - [ ] `npm test` passes
 - [ ] tests are included
 - [ ] documentation is changed or added
-- [ ] contribution guidelines followed [here](CONTRIBUTING.md)
+- [ ] contribution guidelines followed [here](https://github.com/nodejs/citgm/blob/master/CONTRIBUTING.md)
 - [ ] commit message follows commit guidelines