Fixed new_cop.rake suggested path

[dabroz]

rake new_cop[Style/Foobar] suggested to

- Add `require 'lib/rubocop/cop/style/foobar'` into lib/rubocop.rb

while actually paths in lib/rubocop.rb don't start with lib/. This fixes the issue and suggests a correct path instead.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[bbatsov]

👍