This is related to my fix in #658. Apparently, MacOS adds a git
executable that is just a stub which displays an error. This gets past
the try/except I added in #658, and allows all of the GitPython
components to be imported, but since the executable is not *actually*
git, it results in an exception when ``refresh()`` attemepts to run a
``git version``.

Fix small typo and slightly reword docstring.

I found the extra 8 spaces at the start of the examples in the tutorial
to be distracting. The Sphinx dedent option removes these extra spaces
from the rendered code blocks.

I also got a warning about the shell code example not being lexed as
Python, so I converted this to an explicit shell code block.

Using "import as" is normally a time saver but for usability of the documentation, please consider removing osp and join with fully qualified calls for better snippet readability.