Fix examples in docs of mixture.py, bound.py and continuous.py

[sharanry]

Fixes #2806

[ColCarroll]

You need a single blank line after .. code-block:: python (I have been mucking around with rst a lot).

[ColCarroll]

Looks good to me (travis doesn't build docs on PRs) – will merge later today. If you get a chance, you might build the docs locally:

$ cd pymc3 && git checkout mixtureDocs
$ pip install -r requirements.txt
$ pip install -r requirements-dev.txt
$ cd docs/source
$ make html
$ open _build/html/api/distributions/continuous.html

Then post a screenshot if it looks ok?

[sharanry]

@ColCarroll Looks like it is okay.

[ColCarroll]

Thanks, @sharanry !