[GraphX] fixing 3 typos in the graphx programming guide

Corrected 3 Typos in the GraphX programming guide. I hope this is the correct way to contribute.

Author: Benedikt Linse <benedikt.linse@gmail.com>

Closes #4766 from 1123/master and squashes the following commits:

8a63812 [Benedikt Linse] fixing 3 typos in the graphx programming guide