Update README.md

[jehan60188]

added "comma-seperated" the description for env_vars

This is important when using the literal block scalar | with a variable ${{<variable name>}}, as entries are not parsed separately unless they are comma-seperated (newlines aren't enough)

example:

      - id: "deploy"
        uses: "google-github-actions/deploy-cloud-functions@v2"
        with:
          name: "function_name"
          entry_point: "main"
          runtime: "python311"
          memory_mb: "8192MB"
          env_vars: | 
            FOO=${{ env.PROJECT_NAME }}
            BAR=BAZ
          source_dir: ...

sets one environment variable FOO as <project_name>\nBAR=BAZ

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[jehan60188]

I just signed the CLA. Do I need to resubmit this PR?