Fix doc for vmap levels (#50099)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50099
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D25783257
Pulled By: ejguan
fbshipit-source-id: 7d2c7614f87e1c8adc8aefe3fe312b6c98ff6788
