doc: adjust slightly awkward wording in buffer.md

Minor re-wording of three sentences to make them slightly more natural.
Ending sentences with prepositions is fine in many cases, but these
three cases are a bit awkward.

PR-URL: https://github.com/nodejs/node/pull/20037


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>