doc: revise tools/icu/README.md

* Simplify introductory sentence.
* Remove some passive voice.
* Make style in bulleted list a little more consistent.
* Remove superfluous list entry.

PR-URL: https://github.com/nodejs/node/pull/32136


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>