Update README.md to cover mismatched expected master branch

When I tried to run the install with the usual bundle import, I got an error stating that bundle expected "master" branch, but it couldn't be found.

After adding "branch : 'main'" to the import, the installation worked successfully.

Author: claudey

README.md

@@ -24,7 +24,7 @@ Before installing the gem, ensure both your node and [yarn](https://yarnpkg.com/
 
 ```ruby
 # Gemfile
-gem "railsui", github: "getrailsui/railsui"
+gem "railsui", github: "getrailsui/railsui", branch: "main"
 ```
 
 ```bash