bug in antsCorticalThickness

a tiny bug, I had forgotten to add priors directory to -p flag. 

_Please do not merge as I'm testing antsCorticalThickness in large scale and I might come up with some improvements._
@armaneshaghi - i don't think this will work with a node. 

nodes either copy files, link files or use remote files. neither of these cases will generate a directory called priors locally. 

for this interface is at absolutely necessary for the specific location to be called priors, or could the priors be in any directory?