Update CONTRIBUTING.md

Changed "usual steps" link.
Changed  "read this first" link.

docs/CONTRIBUTING.md

@@ -4,8 +4,7 @@
 - Please read our [short and sweet coding guidelines](coding_guidelines.md).
 - For big changes such as adding new feature or refactoring, [file an issue first](https://github.com/Microsoft/AirSim/issues).
 - Use our [recommended development workflow](dev_workflow.md) to make changes and test it.
-- Use [usual steps](https://akrabat.com/the-beginners-guide-to-contributing-to-a-github-project/) to make contributions just like other GitHub projects. If you are not familiar with Git Branch-Rebase-Merge workflow![AAAAAAAAAAAAAAAA](https://user-images.githubusercontent.com/72693627/110688129-88f03380-81e1-11eb-93a4-8a6b6a699588.PNG)
-, please [read this first](https://gist.github.com/markreid/12e7c2203916b93d23c27a263f6091a0).
+- Use [usual steps](https://www.dataschool.io/how-to-contribute-on-github/) to make contributions just like other GitHub projects. If you are not familiar with Git Branch-Rebase-Merge workflow, please [read this first](https://git-rebase.io/).
 
 ## Checklist
 - Use same style and formatting as rest of code even if it's not your preferred one.