doc: fix punctuation and wrapping in buffer.md

Remove unnecessary colons and wrap lines at 80 characters.

PR-URL: https://github.com/nodejs/node/pull/19964


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>