Merge pull request #423 from jegonzal/GraphXProgrammingGuide

Improving the graphx-programming-guide

This PR will track a few minor improvements to the content and formatting of the graphx-programming-guide.

(cherry picked from commit 3fcc68bfa5e9ef4b7abfd5051b6847a833e1ad2f)
Signed-off-by: Reynold Xin <rxin@apache.org>