Add xdg-open section to Miscellaneous

A couple things:

- Don't use `samp` when there is no sample text.
- `/build` is not a great example, because this is an absolute path which will almost certainly never exist on any system. It also won't do what you were intending (try it).
- Consider this:

        ... application. For example::

            xdg-open myfile.pdf
            xdg-open http://definitelyavirus.com/