Fix documentation for git log --since --authored

[freshtonic]

"By default, “git log” displays author dates as “Date” but then uses commit
dates when given a –since option. That seems like broken defaults to me."
- http://www.alexpeattie.com/blog/working-with-dates-in-git/

filter commits by date authored

should be

filter commits by date committed

[matthewmccullough]

Thanks for the contribution. The @github/training-team and I have taken the maintenance of this over so we'll be merging pull requests frequently now. We appreciate your efforts!