Readme / Usage typo

[ptrbrtz]

Hi Josh,
first of all thanks for your packaging plugin.
I started using it today by trying out the code you put in the readme under 'usage'. Being an Scala/SBT newbe it took me half a day to figure out why it was not working.
Problem is, 'archetypes.java_application' should be packageArchetype.java_application'.
But anyway... on the way of finding out I really learned a lot about SBT.
Cheers,
Peter