Updated broken links to GitHub docs in CONTRIBUTING.md

Signed-off-by: Bob Yantosca <[email protected]>

CONTRIBUTING.md

@@ -18,7 +18,7 @@ Submit updates (code and/or data) for mature model developments once you have su
 The practical aspects of submitting code updates are listed below.
 
 ## How can I submit updates?
-We use [GitHub Flow](https://guides.github.com/introduction/flow/index.html), so all changes happen through pull requests. This workflow is [described here](https://guides.github.com/introduction/flow/index.html).
+We use **GitHub Flow**, so all changes happen through [pull requests](https://help.github.com/articles/creating-a-pull-request/). This workflow is [described here](https://docs.github.com/en/get-started/using-github/github-flow).
 
 As the author you are responsible for:
 - Testing your changes