Clean up ddb(4) slightly

• Delete empty Li macro uses [1]. This removes some spaces between the optional command/subcommand arguments.
• Attempt to clarify "show lock" subcommand by being more terse/direct. This addresses an issue with a contraction [2].

MFC after: 1 week
Reported by: make manlint [1], igor [2]
Reviewed by: wblock
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9858