Update SYNOPSIS section to be consistent regarding -u, -i, and -I.

Apparently, when the -u, -i and -I options where added to sed(1), it was
forgotten to add them to both lines in the SYNOPSIS section. They were only
added to the second line, although they apply to both.

With the updated SYNOPSIS, it is now allowed (and consistent) to run:

sed -i BAK s/foo/bar/g some_file

PR: 240556
Submitted by: Oliver Fromme
MFC after: 5 days