change option description to reflect the semantics

At least, for me, 'map' means that there are two values and one is "mapped" to
another.

In this case, just one value is provided (container's port), the other value is
automatically obtained (host's port) and the actual mapping can be seen using
``docker port`` command.
Merge pull request #322 from sa2ajj/port-map-nitpick

change option description to reflect the semantics