Vagrant::Action::Builder RDoc correction

The RDoc comments for `Vagrant::Action::Builder#to_app` reference an
instance of `Vagrant::Action::Environment` as the passed paramter. There
is no class `Vagrant::Action::Environment` available. The param being
passed is actually a `Hash` that represents the "action environment".

This commit corrects the RDoc lines for
`Vagrant::Action::Builder#to_app` to correctly reference the passed
`Hash`.
Merge pull request #1861 from trlinkin/rdoc_builder

Vagrant::Action::Builder RDoc correction