Correct several English spellings

Change-Id: Iee8f1ce242c63e5e96dcc03678eca79c875b28be

I hope to contribute additional changes in the future, but this is my first time creating a fork and a pull request on github, so I thought I'd start simple with nothing but spelling corrections.  I realize Artefact is a British spelling, but the code uses both so I changed them to Artifact for consistency.