Remove $ before shell commands in docs

The developers have a [preference](https://github.com/rtfd/readthedocs.org/pull/4676#discussion_r221400605) to not have a `$` before shell commands in the docs. This makes it easier to copy and paste from our docs. We should remove it everywhere. The following command should show it everywhere.

    grep -Ri "    $ " docs/*.rst docs/*/*.rst