Unify labeling of localPrototype and globalPrototype in Hierarchical models chapter

[mhtess]

Currently, the first code box uses the variable bagToPrototype to refer to the local prototype of the bag.

Code box 2, changes this to colorProb (and drops the prototype label)

Code box 3, prototype comes back but now refers to the global prototype.
From there on out, it looks like prototype refers to the global prototype

[ngoodman]

cleaned up.