doc: Add example of passing a list to axis title

[zxdawn]

Description

The DOC only mentions we can assign title by ax.format(title=title). Actually, we can pass a list to the title and it will set the titles automatically by axs.format(title=titles)

Steps to reproduce

fig, axs = pplt.subplots(ncols=2, nrows=2)

titles = list(map(str, np.arange(4)))

axs.format(title=titles)

Proplot version

Paste the results of import matplotlib; print(matplotlib.__version__); import proplot; print(proplot.version)here.

3.5.1
0.9.5.post202

[zxdawn]

@lukelbd Luke, please feel free to assign me if you don't have time ;) This should be a simple PR.

[zxdawn]

Well, it seems the passing list will be set as all titles now