Fix typos and formatting
Summary: Fix minor typos, sync the text with the current interface, use :guilabel: for the GUI items
Test Plan: none
Reviewers: Krita, Krita: Manual, woltherav
Reviewed By: Krita, Krita: Manual, woltherav
Tags: Krita: Manual
Differential Revision: https://phabricator.kde.org/D20124