Correct various typos in the comments and Mainpage.dox
Summary:
I've been reading through the source code of LibKGAPI to educate myself on programming,
and found a bunch of typos in various comments, as well as Mainpage.dox.
Reviewed By: dvratil
Subscribers: elvisangelaccio, dvratil, KDE PIM
Differential Revision: https://phabricator.kde.org/D12566