Update README.md

Author: socheatsok78

README.md

@@ -118,7 +118,9 @@ This action is designed to be used in GitHub Actions environment. If you want to
 #     curl -fsSL https://raw.githubusercontent.com/flutter-actions/setup-flutter/main/install.sh | bash -s -- <version> <channel>
 # 
 # Example:
-curl -fsSL https://raw.githubusercontent.com/flutter-actions/setup-flutter/main/install.sh | bash -s -- 3.0.2 stable
+
+SETUP_FLUTTER_BRANCH=main
+curl -fsSL https://raw.githubusercontent.com/flutter-actions/setup-flutter/${SETUP_FLUTTER_BRANCH}/install.sh | bash -s -- 3.0.2 stable
 ```
 
 # License