chore: refresh contributing guide to reference the main branch which is the default branch now (#146)

The default branch was renamed to `main`, but our contributing guide is stale.

Author: shivlaks

CONTRIBUTING.md

@@ -42,7 +42,7 @@ Contributions via pull requests are much appreciated.
 
 Before sending us a pull request, please ensure that:
 
-* You are working against the latest source on the *master* branch.
+* You are working against the latest source on the *main* branch.
 * You check the existing open and recently merged pull requests to make sure someone else hasn't already addressed the problem.
 * You open an issue to discuss any significant work - we would hate for your time to be wasted.
 
@@ -182,6 +182,6 @@ If you discover a potential security issue in this project we ask that you notif
 
 ## Licensing
 
-See the [LICENSE](https://github.com/aws/aws-step-functions-data-science-sdk-python/blob/master/LICENSE) file for our project's licensing. We will ask you to confirm the licensing of your contribution.
+See the [LICENSE](https://github.com/aws/aws-step-functions-data-science-sdk-python/blob/main/LICENSE) file for our project's licensing. We will ask you to confirm the licensing of your contribution.
 
 We may ask you to sign a [Contributor License Agreement (CLA)](http://en.wikipedia.org/wiki/Contributor_License_Agreement) for larger changes.