[Fix #1345] Improve offense message for `Rails/RootPathnameMethods`

Fix #1345

The current message is confusing/nonsensical for Dir[]. Let's display the correct code instead to show what needs to be done.

I also added the Dir offenses to the example section.

Before submitting the PR make sure the following are checked:

[x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
[x] Wrote good commit messages.
[x] Commit message starts with [Fix #issue-number] (if the related issue exists).
[x] Feature branch is up-to-date with master (if not - rebase it).
[x] Squashed related commits together.
[x] Added tests.
[x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
[x] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
[ ] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

rubocop / rubocop-rails