tools: disallow trailing whitespace for markdown

markdown had a dispensation because 2 or more trailing spaces triggers a
new paragraph. There are no examples of that usage in Node, all trailing
whitespace found were mistakes, and the dispensation is now removed.

See: https://github.com/nodejs/node/pull/9620
PR-URL: https://github.com/nodejs/node/pull/9676


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>