Update/fix CONTRIBUTING.md URLs

Author: mikemanger

CONTRIBUTING.md

@@ -1,8 +1,8 @@
 # Guidelines for contributing
 
-1. [Fork the repository](https://help.github.com/articles/fork-a-repo).
-2. [Create a topic branch](http://learn.github.com/p/branching.html).
+1. [Fork the repository](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo).
+2. [Create a topic branch](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-branches).
 3. Make your changes, including tests for your changes which maintain [coverage](https://coveralls.io/r/campaignmonitor/createsend-python).
 4. Ensure that all tests pass, by running `rake`.
 5. It should go without saying, but do not increment the version number in your commits.
-6. [Submit a pull request](https://help.github.com/articles/using-pull-requests).
+6. [Submit a pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests).