Update competition readme

[liyiersan]

It seems that Models Genesis is incorporated with nnU-Netv1, could you please update the readme file so that the git repo refers to nnunetv1.
just like this:

git clone -b nnunetv1 https://github.com/MIC-DKFZ/nnUNet.git # v1 branch
cd nnUNet
pip install git+https://github.com/MIC-DKFZ/batchgenerators.git
pip install -e .