fix(PULL_REQUEST_TEMPLATE): use absolute urls

The relative urls do not resolve correctly.

PULL_REQUEST_TEMPLATE.md → .github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,7 @@ TO CONTRIBUTORS:
 
 Make sure you have:
 
- * [ ] reviewed and applied the coding style: [coding](./docs/style.md), [naming](./docs/naming.md)
+ * [ ] reviewed and applied the coding style: [coding](https://github.com/leanprover/mathlib/blob/master/docs/style.md), [naming](https://github.com/leanprover/mathlib/blob/master/docs/naming.md)
  * [ ] for tactics:
      * [ ] added or adapted documentation in [tactics.md](./docs/tactics.md)
      * [ ] write an example of use of the new feature in [tactics.lean](./tests/tactics.lean)
@@ -11,4 +11,4 @@ Make sure you have:
 
 If this PR is related to a discussion on Zulip, please include a link in the discussion.
 
-For reviewers: [code review check list](./docs/code-review.md)
+For reviewers: [code review check list](https://github.com/leanprover/mathlib/blob/master/docs/code-review.md)