Correct Javadoc link to master (#39038)
Summary:
Correct Javadoc link to match the 1.4 version: https://github.com/pytorch/pytorch/blob/release/1.4/docs/source/index.rst
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39038
Differential Revision: D21747969
Pulled By: jlin27
fbshipit-source-id: 941b61204e9be53e15a6351eff6f4935e6a16d24
