update magma instructions (#18410)
Summary:
fixes https://github.com/pytorch/pytorch/issues/18389
cc: stas00
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18410
Differential Revision: D14594198
Pulled By: soumith
fbshipit-source-id: fb46ef77a36c90ad95e47f7066f5d32aa1f1370f
