httpx 0.24 documentation issue for netrc support

[tomchristie]

Thanks to @alexprengere for raising this issue...

Discussed in #2665

^{Originally posted by alexprengere April 18, 2023}
From the docs: https://www.python-httpx.org/advanced/#netrc-support

>>> auth = httpx.NetRCAuth()
>>> client = httpx.Client(auth=auth)

Using that snippet will yield the error:

>>> httpx.NetRCAuth()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: NetRCAuth.__init__() missing 1 required positional argument: 'file'

Using NetRCAuth(file=None) fixes the issue, and produces the expected behavior, but perhaps it should be made the default value (I can open a PR).