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

Doc/library/argparse.rst

@@ -555,7 +555,7 @@ disallowed.
 fromfile_prefix_chars
 ^^^^^^^^^^^^^^^^^^^^^
 
-Sometimes, when dealing with a particularly long argument lists, it
+Sometimes, when dealing with a particularly long argument list, it
 may make sense to keep the list of arguments in a file rather than typing it out
 at the command line.  If the ``fromfile_prefix_chars=`` argument is given to the
 :class:`ArgumentParser` constructor, then arguments that start with any of the