library/apt: consistently use underscores in examples.

To be consistent with the table showing available options, use
underscores in the example tasks, not hyphens, as the table doesn't
list hyphenated versions of option names, so it looks like the
examples could have typos in them.
Merge pull request #1924 from blair/consistly-use-underscores

library/apt: consistently use underscores in examples.