Made edits to the MarkovGraph lab.

I think this lab is in good shape. Two issues worth considering for further revisions down the road:
1. The lab touches on eigenvalues and -vectors, but the course doesn't address these topics until several
weeks later. So it seems a little asynchronous. 
2. I'm not in love with the maze problem, although it's fun. Is there a more real-world-useful problem? The lab 
talks up the importance and applicability of graphs, but all we get is a little toy problem. (I know, there's only so 
much you can do without teaching any of the major graph algorithms.)