docs: Fix the wrong npm package name

[kacetal]

The npm package generate-type-guards doesn't exist.

npm error code E404
npm error 404 Not Found - GET https://registry.npmjs.org/generate-type-guards - Not found
npm error 404
npm error 404  'generate-type-guards@*' is not in this registry.
npm error 404
npm error 404 Note that you can also install from a
npm error 404 tarball, folder, http url, or git url.