Fix NameError in stochastic_volatility.ipynb

When this notebook was added back in 85a017b (after being deleted in
138e145), a few names did not get updated to reflect the change of
`from pymc import *` to `import pymc as pm`.
Merge pull request #463 from kyleam/fix-stochastic_volatility-nameerror

Fix NameError in stochastic_volatility.ipynb