gh-90795: Fix error message incorrectly including "positional"

[StanFromIreland]

This does not cover the more complicated case of a positional only and keyword-or-positional argument, that remains the same:

>>> def foo(a, /, b): pass
... 
>>> foo()
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    foo()
    ~~~^^
TypeError: foo() missing 2 required positional arguments: 'a' and 'b'

• Issue: Incorrect error message: "missing 1 required positional argument" #90795

[StanFromIreland]

Friendly ping @markshannon & @ericvsmith :-)