Fix capitalization in Overcommit instructions
Fix capitalization in Overcommit instructions

This section was added by b2176b91, which reflected the correct way to
configure Overcommit to work with RuboCop at the time. However, since
then, Overcommit changed the capitalization of the hook name and
configuration to match the capitalization preferred by the RuboCop
project[0], so we need to update these instructions to match.

[0]: https://github.com/brigade/overcommit/commit/68a000f1

jonas054/rubocop,jonas054/rubocop