PR template - remove pull requests link full stop (#115090)

.github/pull_request_template.md

 <!-- Thank you for submitting a Pull Request. Please:
 * Read our Pull Request guidelines:
-  https://github.com/microsoft/vscode/wiki/How-to-Contribute#pull-requests.
+  https://github.com/microsoft/vscode/wiki/How-to-Contribute#pull-requests
 * Associate an issue with the Pull Request.
 * Ensure that the code is up-to-date with the `master` branch.
 * Include a description of the proposed changes and how to test them.