Replace the broken link about pull request (PowerShell#16771)

Author: daxian-dbw

docs/maintainers/README.md

@@ -6,8 +6,8 @@ One of their primary responsibilities is merging pull requests after all require
 They have [write access](https://docs.github.com/en/free-pro-team@latest/github/setting-up-and-managing-organizations-and-teams/repository-permission-levels-for-an-organization) to the PowerShell repositories which gives them the power to:
 
 1. `git push` to the official PowerShell repository
-1. Merge [pull requests](https://www.thinkful.com/learn/github-pull-request-tutorial/)
-1. Assign labels, milestones, and people to [issues](https://guides.github.com/features/issues/) and [pull requests](https://www.thinkful.com/learn/github-pull-request-tutorial/)
+1. Merge [pull requests](https://docs.github.com/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests)
+1. Assign labels, milestones, and people to [issues](https://guides.github.com/features/issues/) and [pull requests](https://docs.github.com/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests)
 
 ## Table of Contents