 add `API: true` to GetUsage() in cmd_pgp_list.go
 add `API: true` to GetUsage() in cmd_pgp_list.go

This fixes https://keybase.atlassian.net/browse/CORE-2931.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client