Update README.md

Fix refs from master to main

Author: eroullit

README.md

@@ -66,7 +66,7 @@ If you get a permission denied message, you might have to exclude the directory
 
 [![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/rust-lang/rustlings)
 
-[![Open Rustlings On Codespaces](https://github.com/codespaces/badge.svg)](https://github.com/codespaces/new/?repo=rust-lang%2Frustlings&ref=master)
+[![Open Rustlings On Codespaces](https://github.com/codespaces/badge.svg)](https://github.com/codespaces/new/?repo=rust-lang%2Frustlings&ref=main)
 
 ## Manually