chore: Update README.md. Fix --set-env-var to --set-env-vars

[sbleru]

Description

This error happened, so fix option.

ERROR: (gcloud.run.deploy) unrecognized arguments:
  --set-env-var (did you mean '--set-env-vars'?)

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Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

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[pattishin]

Appreciate the change - will need to close out due to the absence of a cla.