gh-36460: add default legend_color to eliminate KeyError when plotting arrow2d or disk
Fixes #36153.
The line `g._legend_colors = [options['legend_color']]` appears in the
code for seven graphics objects. Five of them (`circle`, `ellipse`,
`line2d`, `point2d`, and `polygon2d`) set `None` as the default value
for `legend_color`. The other two (`arrow2d` and `disk`) do not set a
default, so a `KeyError` is raised if a `legend_label` is supplied, but
no `legend_color` is specified. This bug was pointed out in #36153.
This PR adds `None` as the default value of `legend_color` in `arrow2d`
and `disk`.
It also adds the corresponding doctest to all seven of these graphics
objects, except `line2d`, which already has the example
`line([(0,0),(1,1)], legend_label='line')`.
The PR also corrects a typo in the docstring of the `cone` graphic
object.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
URL: https://github.com/sagemath/sage/pull/36460
Reported by: DaveWitteMorris
Reviewer(s): Kwankyu Lee
