Update PULL_REQUEST_TEMPLATE.md

Author: hippke

PULL_REQUEST_TEMPLATE.md

@@ -2,5 +2,5 @@ Thank you for your contribution to TLS.
 Before submitting this PR, please make sure:
 
 - [ ] Your code builds clean without any errors or warnings
-- [ ] [Existing test cases](https://github.com/hippke/tls/blob/master/transitleastsquares/tests.py) work correctly
+- [ ] [Existing test cases](https://github.com/hippke/tls/tree/master/transitleastsquares/tests) work correctly
 - [ ] New test cases have been added to cover the additional and/or changed functionality