Fix prod_or_not in example-static

[olivercoad]

Currently this step silently fails with

✔ production or not
Run if [$REF == 'refs/heads/master']
  if [$REF == 'refs/heads/master']
  then
      echo "::set-output name=vercel-args::--prod"
  else
      echo "::set-output name=vercel-args::"
  fi
  shell: /bin/bash -e {0}
  env:
    VERCEL_ORG_ID: ***
    VERCEL_PROJECT_ID: ***
    REF: refs/pull/36/merge
/home/runner/work/_temp/ef8d826e-e24a-4b54-a019-534ae6281380.sh: line 1: [refs/pull/36/merge: No such file or directory

[sonarcloud]

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