pull-request-template: broken link

[alexcg1]

When I use gh pr create to create a pull request and choose the pull-request-template.md, the final line is:

- [ ] check and update documentation. See [guide](https://github.com/jina-ai/jina/CONTRIBUTING.md#documentation-guidelines) and ask the team.

The link is dead. It simply points to a page that displays the text "Not Found"

Fix

Change link to https://github.com/jina-ai/jina/blob/master/CONTRIBUTING.md#-contributing-documentation