Fix formatting and minor typos
Summary: Just minor fixes: a -> an, shit -> shirt, fix heading, etc.
Test Plan: none
Reviewers: Krita, Krita: Manual, woltherav
Reviewed By: Krita, Krita: Manual, woltherav
Subscribers: alvinhochun, woltherav, rempt
Tags: Krita: Manual
Differential Revision: https://phabricator.kde.org/D19807