Fix "rack-gateway scale" command to accept a service arg

[ndbroadbent]

❯ cx scale --help
scale app processes

Usage:
rack-gateway scale [flags]

Flags:
-a, --app string app name (auto-detected from .convox/app or current directory)
--count int number of processes
--cpu int cpu allocation
-h, --help help for scale
--memory int memory allocation

Global Flags:
--api-token string API token for CLI requests (overrides RACK_GATEWAY_API_TOKEN)
--config string Config directory (default "/Users/ndbroadbent/.config/rack-gateway")
--mfa-code string MFA code (TOTP/Yubikey/backup) for step-up authentication
--mfa-method string MFA method to use (totp or webauthn)
-r, --rack string Rack to use (overrides current rack)