Correct grammer in MAINTAINERS.md

Some of these are actual grammatical corrections, and some (like capitalizing Docker) are just style changes for consistency. Some of the changes are hard to see because of the line wrapping, but ```git diff --color-words```, highlights them nicely.
Google seems to consider "roadmap" as a valid single word: https://www.google.com/search?q=define%3Aroadmap