chore(changeset): trim the route-groups changeset to one line (#4143)

The changeset body is copied verbatim into the changelog, where the
rationale paragraphs from #4070 are noise. Keep the line that says what
changed and where it matters; the reasoning is already in the commit,
the PR and issue #3991.

This was reviewed on #4070 and fixed on #4140, a CI mirror of that PR.
#4070 is the one that merged and #4140 was closed, so the fix never
landed. The changeset is still unconsumed on `main`, so correcting the
file is enough — the open release PR regenerates from it and no
CHANGELOG.md has been written yet.

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Pranay Prakash <1797812+pranaygp@users.noreply.github.com>