update username flag help text
update username flag help text

Signed-off-by: Siddhartha Karthik Copesetty <da58b0c134ced9fa3847c7d85a083541cd9a0663@docker.com>

jfrazelle/notary,docker/notary,jfrazelle/notary,johnsandiford/notary,johnsandiford/notary,HuKeping/notary,endophage/notary,docker/notary,HuKeping/notary,endophage/notary,jfrazelle/notary,HuKeping/notary,docker/notary,endophage/notary,johnsandiford/notary