README: Environment variable typo fix (#11)

README.md

@@ -3,7 +3,7 @@
 This action checks out your repository so that your workflow operates from the root of the repository
 
 By default, this is equivalent to running `git fetch` and `git checkout $GITHUB_SHA`, so that you'll always have your repo contents at the version that triggered the workflow.
-See [here](https://help.github.com/en/articles/events-that-trigger-workflows) to learn what `$GITHUBS_SHA` is for different kinds of events.
+See [here](https://help.github.com/en/articles/events-that-trigger-workflows) to learn what `$GITHUB_SHA` is for different kinds of events.
 
 # Usage