Fix 0.22.3 release notes heading

Summary:
D7544 should have included this change in the staging file (doc/release-notes.md).
It included it with the template, but that only affects new release notes.

Test Plan: Read it.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7673