Fix Dotty CI badge

Author: nicolasstucki

README.md

@@ -1,6 +1,6 @@
 Dotty
 =====
-[![Dotty CI](https://github.com/scala/scala3/workflows/Dotty/badge.svg?branch=master)](https://github.com/scala/scala3/actions?query=branch%3Amain)
+[![Dotty CI](https://github.com/scala/scala3/workflows/Dotty/badge.svg?branch=main)](https://github.com/scala/scala3/actions?query=branch%3Amain)
 [![Join the chat at https://discord.com/invite/scala](https://img.shields.io/discord/632150470000902164)](https://discord.com/invite/scala)
 
 * [Documentation](https://docs.scala-lang.org/scala3/)