Remove wrong usage of :option:

From http://www.sphinx-doc.org/en/stable/domains.html#directive-option:

  > The directive will create cross-reference targets for the given options,
  > referencable by option (in the example case, you’d use something like
  > :option:`dest_dir`, :option:`-m`, or :option:`--module`).

I don't think we're intending here to create cross references,
we simply want to render it nicely.

So just use code formatting instead.

This gets rid of the last build warnings.