Fix ResNet50 docstring.
Fix ResNet50 docstring.

resnet_v2 is set to `False` by default but described as `True` in docstring.

PiperOrigin-RevId: 272209667
Change-Id: Ia11ada33e5853dbe22ca2ad962bcc4eb4e84e924

deepmind/sonnet,deepmind/sonnet