Update README.md headers settable field casing

[robertvanhoesel]

It is a bit of a gotcha that the headers settable fields are the lowercase counterparts to their Header. The docs incorrectly mention the case sensitive varitions, which when followed causes an error:

 - uses: google-github-actions/upload-cloud-storage@v1
       with:
          path: /foo
          destination: bucket
          headers: |-
            Cache-Control: public, max-age=31536000, no-transform, immutable

google-github-actions/upload-cloud-storage failed with:
invalid header key "Cache-Control" - custom header keys must be prefixed with "x-goog-meta-"

For future users and clarity, I think it is best to update the documentation to use the fully lowercase versions.

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