The default branch for ruby-getting-started is 'main', not 'master

- Ensure the user is using the master branch to match later instructions to do a git push master.

docs/deployment/application-deployment.md

@@ -9,7 +9,7 @@ Once you have configured Dokku with at least one user, you can deploy applicatio
 ```shell
 # from your local machine
 # SSH access to github must be enabled on this host
-git clone https://github.com/heroku/ruby-getting-started
+git clone https://github.com/heroku/ruby-getting-started --branch master
 ```
 
 ### Create the app