Fix mismatch in README.md Docker Image section (#59199)
Summary:
docker.Makefile has CUDNN_VERSION=8 as the defaults, but README.md states cuDNN v7
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59199
Reviewed By: mruberry
Differential Revision: D28808611
Pulled By: ngimel
fbshipit-source-id: 96cea32bfe33184b2bff69b7bb7f3e50a2b9c6aa
