docs: change default branch from master to main (#397)

Author: peaceiris

README.md

@@ -1,7 +1,7 @@
-[![license](https://img.shields.io/github/license/peaceiris/actions-gh-pages.svg)](https://github.com/peaceiris/actions-gh-pages/blob/master/LICENSE)
+[![license](https://img.shields.io/github/license/peaceiris/actions-gh-pages.svg)](https://github.com/peaceiris/actions-gh-pages/blob/main/LICENSE)
 [![release](https://img.shields.io/github/release/peaceiris/actions-gh-pages.svg)](https://github.com/peaceiris/actions-gh-pages/releases/latest)
 [![GitHub release date](https://img.shields.io/github/release-date/peaceiris/actions-gh-pages.svg)](https://github.com/peaceiris/actions-gh-pages/releases)
-![Test](https://github.com/peaceiris/actions-gh-pages/workflows/Test/badge.svg?branch=master&event=push)
+![Test](https://github.com/peaceiris/actions-gh-pages/workflows/Test/badge.svg?branch=main&event=push)
 [![CodeFactor](https://www.codefactor.io/repository/github/peaceiris/actions-gh-pages/badge)](https://www.codefactor.io/repository/github/peaceiris/actions-gh-pages)
 
 <img width="400" alt="GitHub Actions for deploying to GitHub Pages with Static Site Generators" src="./images/ogp.svg">
@@ -1081,7 +1081,7 @@ jobs:
 
 - [MIT License - peaceiris/actions-gh-pages]
 
-[MIT License - peaceiris/actions-gh-pages]: https://github.com/peaceiris/actions-gh-pages/blob/master/LICENSE
+[MIT License - peaceiris/actions-gh-pages]: https://github.com/peaceiris/actions-gh-pages/blob/main/LICENSE