Improve 'Usage' docs for upgrade command.
Improve 'Usage' docs for upgrade command.

Before: Usage: [command] [flags]
After:  Usage: upgrade [flags]

andreypopp/esy,ConAntonakos/yarn,seripap/emojiless-yarn,CHECK24/yarn,mourner/yarn,seripap/emojiless-yarn,bsansouci/esy,bsansouci/esy,troydemonbreun/yarn,mvestergaard/yarn,mvestergaard/yarn,mgcrea/yarn,kasperlewau/yarn,andreypopp/esy,lawliet89/yarn,suitupalex/yarn,mourner/yarn,lawliet89/yarn,joeyespo/yarn,joeyespo/yarn,bsansouci/esy,troydemonbreun/yarn,MiguelAlho/yarn,mgcrea/yarn,kasperlewau/yarn,MiguelAlho/yarn,dguo/yarn,suitupalex/yarn,ConAntonakos/yarn,CHECK24/yarn,andreypopp/esy,dguo/yarn,onemen/yarn,onemen/yarn