sfdisk: (man page) use comma between short and long option instead of "or"

sfdisk: (man page) use comma between short and long option instead of "or"

Also fix some erratic formatting, indent the third example too, drop
the trailing % prompt in examples, add a blank line before and after,
and improve the wording here and there.

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>