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.
Webster's has "road map" as two words, so does AP. I think it's one of those terms that is evolving, so both are probably acceptable. Generally, I prefer that we hew to AP style, however.