Update PULL_REQUEST_TEMPLATE.md (#238)

Fixed a broken link in the PR template.

Co-authored-by: Jen Bradley <55467578+janbridley@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -22,4 +22,4 @@
 - [ ] I am familiar with the [**Contributing Guidelines**](https://github.com/glotzerlab/coxeter/blob/master/CONTRIBUTING.md).
 - [ ] I agree with the terms of the [**Contributor Agreement**](https://github.com/glotzerlab/coxeter/blob/master/ContributorAgreement.md).
 - [ ] The changes introduced by this pull request are covered by existing or newly introduced tests.
-- [ ] I have updated the [changelog](https://github.com/glotzerlab/coxeter/blob/master/ChangeLog.txt) and the [credits](https://github.com/glotzerlab/coxeter/blob/master/doc/source/credits.rst).
+- [ ] I have updated the [changelog](https://github.com/glotzerlab/coxeter/blob/master/ChangeLog.rst) and the [credits](https://github.com/glotzerlab/coxeter/blob/master/doc/source/credits.rst).