[Feat] doc: improve python example when a required parameter has multiple options but no default

[petrasovaa]

In the new documentation of v.generalize the method option shows "string":

gs.run_command("v.generalize", input="name", output="name", method="string", threshold=float)

which is not a valid option.

Describe the solution you'd like
It would pick e.g. the first option, so that the example runs.

(The threshold is not set, so it won't run, but that's a separate issue.)