doc: improve PR-review paragraph in CONTRIBUTING.md

* Remove redundant "blocking it or stopping it" as blocking and stopping
  are the same thing in this case.
* Make another sentence less wordy. Fix incorrect verb conjugation.
  Break into two clear sentences.

PR-URL: https://github.com/nodejs/node/pull/17931


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>