Change step with new_cop command for rubocop-rspec_rails

[ydah]

before:

Do 4 steps:
  1. Modify the description of RSpecRails/CopName in config/default.yml
  2. Implement your new cop in the generated file!
  3. Commit your new cop with a message such as
     e.g. "Add new `RSpecRails/CopName` cop"
  4. Run `bundle exec rake changelog:new` to generate a changelog entry

after:

Do 4 steps:
  1. Modify the description of RSpecRails/CopName in config/default.yml
  2. Implement your new cop in the generated file!
  3. Commit your new cop with a message such as
     e.g. "Add new `RSpecRails/CopName` cop"
  4. Add an entry about new cop to CHANGELOG.md

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• [-] Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[pirj]

Thanks!