[doc] Cleanup trailing whitespaces

Summary: D17597 introduced a lot of lines with trailing whitespace and this shows up in git diff. Took this opportunity to cleanup one other trailing whitespace in an old release note.

Test Plan: read it

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17602