[balooctl] Add missing help for "config set", normalize string

Summary:
The "contentIndexing" command was not listed for "balooctl config set",
although valid.
Change the help string to match the remaining commands, remove trailing
full stop and capitalize "Baloo".

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18827