Fix path error in the README section about frontend schema generation
Fix path error in the README section about frontend schema generation

- The `bundle exec ./bin/generate_frontend_schema
  formats/case_study/publisher/schema.json >
  formats/case_study/frontend/schema.json` example, adapted for our
  use in contacts, line errored with
  "formats/contact/publisher/schema.json: no such file or directory"
  because `schema.json` is a built file and therefore is only in the
  `dist/` directory. Change the README example to refer to `dist/...`.

alphagov/govuk-content-schemas,alphagov/govuk-content-schemas