chore(docs): fix CONTRIBUTING.md gitpod link (#18556)

The current link to the gitpod section does nothing. This will fix the link to the correct section of CONTRIBUTING.md

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -46,7 +46,7 @@ let us know if it's not up-to-date (even better, submit a PR with your  correcti
 
 The following steps describe how to set up the AWS CDK repository on your local machine.
 The alternative is to use [Gitpod](https://www.gitpod.io/), a Cloud IDE for your development.
-See [Gitpod section](#gitpod) on how to set up the CDK repo on Gitpod.
+See [Gitpod section](#gitpod-alternative) on how to set up the CDK repo on Gitpod.
 
 ### Setup