Docs: Correct command in examples of adding packages with pnpm

[scottdotweb]

What I did

Changed instances in the docs of pnpm install to pnpm add, which is the correct command to use when adding packages with pnpm.

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Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

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  • bug: Internal changes that fixes incorrect behavior.
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[jonniebigodes]

@scottdotjs, thanks for putting together this pull request and helping us improve the documentation by catching these minor but important typos in the examples. We appreciate it 🙏 !

I've checked, and everything is good on my end. I'll merge this once the checklist clears.

Hope you have a fantastic day.

Stay safe