Fix link to list of contributors. (#225)

.github/PULL_REQUEST_TEMPLATE.md

@@ -21,7 +21,7 @@
 <!-- If you are unsure about any of these items, do not hesitate to ask! -->
 - [ ] I am familiar with the [**Contributing Guidelines**](https://github.com/glotzerlab/signac/blob/master/CONTRIBUTING.md).
 - [ ] I agree with the terms of the [**Contributor Agreement**](https://github.com/glotzerlab/signac/blob/master/ContributorAgreement.md).
-- [ ] My name is on the [list of contributors](https://github.com/glotzerlab/signac/blob/master/contributors.txt).
+- [ ] My name is on the [list of contributors](https://github.com/glotzerlab/signac/blob/master/contributors.yaml).
 - [ ] My code follows the [code style guideline](https://github.com/glotzerlab/signac/blob/master/CONTRIBUTING.md#code-style) of this project.
 - [ ] The changes introduced by this pull request are covered by existing or newly introduced tests.