Update `admins remove` command docs

[ipmb]

It accepts multiple email addresses. See how admins add is written:

apppack/cmd/admins.go

Lines 83 to 88 in 3b0c36b

	Use: "add <email>...",
	Short: "add administrators to the account",
	Long: "*Requires admin permissions.*\nUpdates the account Cloudformation stack to add administrators.",
	DisableFlagsInUseLine: true,
	Args: cobra.MinimumNArgs(1),
	Example: "apppack admins add user1@example.com user2@example.com",