updated readme to use main as default branch

The actual default fails to install as per the current `README.md` this is needed until `mint` team support `main` as alternative to `master` 

Refer: 

XcodeMigrate#29
yonaskolb/Mint#204

Author: palaniraja

README.md

@@ -28,7 +28,7 @@ However, migrating existing (large) Xcode project to Bazel is not easy. It requi
 ## Usage
 
 ```swift
-mint install XcodeMigrate/XcodeMigrate
+mint install XcodeMigrate/XcodeMigrate@main
 mint run xcode-migrate generate -p /path/to/your.xcodeproj
 ```