[3.12] tarfile: Fix positional-only syntax in docs (GH-105770) (#105773)

The syntax used in the current docs (a / before any args) is invalid.

I think the right approach is for the arguments to arbitrary
filter functions to be treated as positional-only, meaning that users
can supply filter functions with any names for the argument. tarfile.py
only calls the filter function with positional arguments.
(cherry picked from commit 5cdd5ba49db10f05e204e7a49ce184222a93dce8)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>