Example code in Mixture docs does not display correctly

[twiecki]

http://docs.pymc.io/api/distributions/mixture.html
PR: #2799
cc @ericmjl

[sharanry]

Hey, could I take this up?

[fonnesbeck]

@sharanry feel free to!

[sharanry]

@twiecki just realized .. code-block:: python is not added for many more example codes in bound.py too. I will change that too?

[twiecki]

Yes, that'd be great!

[ericmjl]

@sharanry yay! Thank you for contributing!! 😄