Improve "Checkins over time" image

[juan serrano]

I think it would make more sense that under the section 'What is a "commit"?' the image (png) "Checkins over time" should have the numbers in the "Version" (top row) match up with the commit files. For example under, "Version 2" we have commits/files labeled "A1" and "C1".
I think it would be more intuitive and clearer for the reader to have the numbers be the same.