Fix pnpm install command

[levrik]

According to https://pnpm.io/cli/install pnpm install is used to install a whole project and not to add a single dependency. pnpm add should be used instead. I just tried to use pnpm install like shown here and it surprisingly worked but since this is not documented it might be better to use pnpm add in Rome's documentation.

[netlify[bot]]

✅ Deploy Preview for docs-rometools ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	c21e3ff625d755fd6fcce77d6611e1828ebb63ba
🔍 Latest deploy log	https://app.netlify.com/sites/docs-rometools/deploys/636fcea0cefa8d000abbe899
😎 Deploy Preview	https://deploy-preview-3688--docs-rometools.netlify.app/guides/getting-started
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[sebmck]

Thank you!