[Chore] Fix link to style guide

Problem: the link to a style guide in pull request template is broken.

Solution: we seem to never have style guide in this repo, so link to the
style-guide in the serokell/metatemplates repository.

.github/pull_request_template.md

@@ -34,7 +34,7 @@ you must be able to justify that.
 
 - [ ] Style
   - My commits comply with [the policy used in Serokell](https://serokell.notion.site/Commit-and-PR-policy-4cf98e1a910a415d86b5f2491d9af1af).
-  - My code complies with the [style guide](../tree/master/docs/code-style.md).
+  - My code complies with the [style guide](https://github.com/serokell/style/blob/master/haskell.md).
   - Each commit of this pull request contains a description.
     - For new features, this description contains the use case for the new functionality.
     - For bug fixes, it describes both the attacked problem and a solution for it.