FVM - Improve fvm install help command

[EstebanBorai]

Currently FVM's fvm install -h/--help command renders the following text.

% fvm install -h
Install a Fluvio Version

Usage: 

Arguments:
  [VERSION]  Version to install [default: stable]

Options:
      --registry <REGISTRY>  Registry used to fetch Fluvio Versions [env: HUB_REGISTRY_URL=] [default: https://hub.infinyon.cloud/]
  -h, --help                 Print help

Instead the command fvm install -h/--help should render:

[VERSION]  Version to install: stable, latest, or named-version x.y.z [default: stable]