doc: edit BUILDING.md

Minor edits to BUILDING.md to keep sentences short and clear.

PR-URL: https://github.com/nodejs/node/pull/24243


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>