Update action.yml

This partially fixes #940 by updating the description to match the request. However I'm assuming the runner adds in the `Action` part by truncating the description? cc @bryanmacfarlane

action.yml

@@ -1,5 +1,5 @@
 name: 'Checkout'
-description: 'Get sources from a GitHub repository.'
+description: 'Checkout a Git repository'
 inputs: 
   repository:
     description: 'Repository name'