ReleaseProcessOverview.md: fix Step 11's sed invocation for
BSD/macOS portability.
Hit for real during SDP 2026.1 GA release Step 14: Step 11's
`sed -i -E -e ...` on macOS treats '-E' as sed -i's backup-suffix
argument (BSD sed requires one, even empty, and consumes whatever
follows -i as it), leaving 15 stray '*.adoc-E' backup files with
the pre-edit content. These then surfaced as bogus 'opened for add'
candidates when Step 14's p4 rec ran across the whole doc tree.
Cleaned up the stray files (reverted the accidental adds, deleted
from disk) before proceeding.
Replaced with a redirect-to-temp-file-then-mv pattern, which works
identically on GNU and BSD sed and needs no backup-suffix handling
at all. The original patterns don't actually need extended regex,
so dropping '-E' entirely was also safe.