docs: fix pnpm dev install command

[davidavz]

This PR corrects the pnpm installation command in the documentation.

Previously the docs suggested using pnpm install --dev, which is invalid.

With pnpm, the correct syntax is:

pnpm add --save-dev <package>

This aligns the docs with pnpm’s CLI conventions and ensures users can
successfully install devDependencies.

[vercel]

@davidavz is attempting to deploy a commit to the nativewind Team on Vercel.

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