Fix misleading doc string in quint8.h (#48418)
Summary:
The doc string let suppose that `quint8` is for singed 8 bit values.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48418
Reviewed By: ngimel
Differential Revision: D25181705
Pulled By: mrshenli
fbshipit-source-id: 70e151b6279fef75505f80a7b0cd50032b4f1008
