Fix typo in NestedGroups doc

[ybiquitous]

This change aims to fix just a typo in the RSpec/NestedGroups document.
See https://docs.rubocop.org/rubocop-rspec/cops_rspec.html#rspecnestedgroups

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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).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded in default/config.yml to the next minor version.

If you have modified an existing cop's configuration options:

• Set VersionChanged in config/default.yml to the next major version.

[pirj]

Thanks!