Typo fixes for documentation.

From: Andrew Ash <andrew@andrewash.com>
Review: https://reviews.apache.org/r/10002

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1463805 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' into testing