Fix Contribution Guidelines url in PR Template (#2308)

The link in the Pull request template was invalid, this corrects it.

Author: hamiltonkibbe

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 <!-- Thank you for your contribution! The following items must be addressed before the code can be merged. Please don't hesitate to ask for help if you're unsure of how to accomplish any of the items. Feel free to remove checklist items that are not relevant to your change. -->
 
  - [ ] Closes #xxxx
- - [ ] I am familiar with the [contributing guidelines](https://pvlib-python.readthedocs.io/en/latest/contributing.html)
+ - [ ] I am familiar with the [contributing guidelines](https://pvlib-python.readthedocs.io/en/latest/contributing/index.html)
  - [ ] Tests added
  - [ ] Updates entries in [`docs/sphinx/source/reference`](https://github.com/pvlib/pvlib-python/blob/main/docs/sphinx/source/reference) for API changes.
  - [ ] Adds description and name entries in the appropriate "what's new" file in [`docs/sphinx/source/whatsnew`](https://github.com/pvlib/pvlib-python/tree/main/docs/sphinx/source/whatsnew) for all changes. Includes link to the GitHub Issue with `` :issue:`num` `` or this Pull Request with `` :pull:`num` ``. Includes contributor name and/or GitHub username (link with `` :ghuser:`user` ``).