manuals: Fix typos in -offset for .Bd and .Bl

The intended value is:

-offset indent

If there's any typo such that the value doesn't match the pre-defined
strings, then the offset is the same width as the value. So by chance,
"-offset -ident" ended up being a standard-width indent (since the
default indent is 6 chars, and "-ident" also has 6 chars), whereas
"-offset -indent" had a longer indent, and "-offset ident" had a shorter
one.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne, Alexander Ziaee <concussious.bugzilla@runbox.com>
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1436