documentation: engine mounting syntax has changed in rails 3.2.8

[bogn]

Had to change the mount line from the Readme from

mount Translator::Engine, :to => '/admin'

to the syntax that is shown in the comments of actionpack/lib/action_dispatch/routing/mapper.rb#L396

mount Translator::Engine => '/admin'

to make it work with rails 3.2.8. Otherwise the error "A rack application must be specified" was thrown.

[hubertlepicki]

this needs updating in documentation indeed.