docs: use ref to manifest schema for initFields property of yarnrc schema

[ludmila-lovelace]

What's the problem this PR addresses?

Validating a .yarnrc.yml file that contains the following:

initFields:
  files:
    - dist/**
  dependencies:
    react: ^17.0.2

fails, as the only expected types are string, number, or boolean. However, this configuration is valid and already works with yarn init.

How did you fix it?

Modified the expected type of initFields to be equivalent to the manifest.json schema.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.