PR template: Fix broken link to contributing.md

The link is broken on actual PR pages.
Not using relative link, as it would be github-specific anyway, and the relative link would be broken on some other pages.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 # Pull request title
 
-- [ ] I read [contributing guideline](.github/CONTRIBUTING.md)
+- [ ] I read [contributing guideline](https://github.com/deezer/spleeter/blob/master/.github/CONTRIBUTING.md)
 - [ ] I didn't find a similar pull request already open.
 - [ ] My PR is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)