Fixes the tau random variable thrice

[joshdevins]

The example of a PyMC stochastic variable was using tau three times and not the two lambda variables at all. This makes it clearer that they are all PyMC stochastic variables.

[CamDavidsonPilon]

The point of this line was to show that calling .random() would indeed return different results each time. Could you change it so there are three print statements?

print tau.random(), tau.random(), ...
print lambda_1.random(), ...
...