Fix README error

[dslatkin]

I was using the example from the README to setup an async script in a workflow and got this error from the API:

Unhandled error: HttpError: No commit found for SHA: undefined

Digging a bit deeper, I discovered that process.env.SHA didn't exist, and should probably be GITHUB_SHA instead. Swapping it out to this worked.

[joshmgross]

I believe this example was intending the SHA to be set as an input

      - uses: actions/github-script@v6
        env:
          SHA: '${{env.parentSHA}}'
        with:
          script: |
            const script = require('./path/to/script.js')
            await script({github, context, core})

So process.env.SHA wasn't meant to directly match an existing variable in the context like GITHUB_SHA

[dslatkin]

That makes sense. I'm not sure I see parentSHA defined anywhere, either. Would it make sense to remove lines 341-342 in that case as well to reduce confusion? Thank you for reviewing!

[dslatkin]

That makes sense. I'm not sure I see parentSHA defined anywhere, either. Would it make sense to remove lines 341-342 in that case as well to reduce confusion? Thank you for reviewing!

Just a heads up, I did just remove these lines in a new commit.