sdiff: Fix usage message.

The --ignore-all-space option was incorrectly documented as
--ignore-all-spaces.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: 0mp, markj
Differential Revision: https://reviews.freebsd.org/D46161

(cherry picked from commit ca75b7dac886de66fc06fd31facfa2c561f7567e)