Fix AGIC typo and remove preview label from VN

[seanmck]

Help text changes only

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/seanmck/azure-cli-extensions.git@aks-help-cleanup#subdirectory=src/$EXT&egg=$EXT"