fix a couple of typos in README markdown (#20819)
Summary:
was reading the README on github and came across a couple of typos.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20819
Differential Revision: D15469603
Pulled By: nairbv
fbshipit-source-id: 0ed7868de2d4e6d82557a8c170783966f8a1afd7
