Fix typo in argparse docs. (GH-92691)

# Fix typo in argparse docs.

> Sometimes, when dealing with **a** particularly long argument list**s**, [...]

Mixture between plural and singular forms is incorrect. Use singular consistently since typically only a single argument list is employed. Change to:

> Sometimes, when dealing with a particularly long argument list, [...]

No issue was opened, since this is a trivial change.

Automerge-Triggered-By: GH:rhettinger