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

Tags: #kde_pim, #libkgapi

Differential Revision: https://phabricator.kde.org/D12566