update downloadIstio script to refer to ambient

[craigbox]

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   101  100   101    0     0    181      0 --:--:-- --:--:-- --:--:--   181
100  4899  100  4899    0     0   5904      0 --:--:-- --:--:-- --:--:--  5904

Downloading istio-1.22.0-beta.0 from https://github.com/istio/istio/releases/download/1.22.0-beta.0/istio-1.22.0-beta.0-osx.tar.gz ...
Istio 1.22.0-beta.0 Download Complete!

Istio has been successfully downloaded into the istio-1.22.0-beta.0 folder on your system.

Next Steps:
See https://istio.io/latest/docs/setup/install/ to add Istio to your Kubernetes cluster.

To configure the istioctl client tool for your workstation,
add the /tmp/istio/istio-1.22.0-beta.0/bin directory to your environment path variable with:
	 export PATH="$PATH:/tmp/istio/istio-1.22.0-beta.0/bin"

Begin the Istio pre-installation check by running:
	 istioctl x precheck 

Need more information? Visit https://istio.io/latest/docs/setup/install/ 

Add something here to tell people that they can try ambient mode, and point them at the documentation.