Incorrect reference to CDANN paper in Readme

[divyat09]

I suppose the reference to the CDANN paper in Readme is wrong. It points to the following paper (https://arxiv.org/abs/1807.08479), which actually uses MMD in their method, and not domain discriminators that are used in the implementation of CDANN in DomainBed (code).

I think the correct reference to the approach CDANN should be the following paper, which uses domain discriminators.

Ya Li, Xinmei Tian, Mingming Gong, Yajing Liu, Tongliang Liu, Kun Zhang, and Dacheng Tao. Deep domain generalization via conditional invariant adversarial networks.ECCV, 2018d (Link).