Fix section about Getting the Admin Portal URL

Doc 2.6 about installing 3scale using operator, on section
"2.7.3. Getting the Admin Portal URL"
it says to use "oc get tenants" command to get admin portal url.

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.6/html/installing_3scale/onpremises-installation#deploying-threescale-using-the-operator

This is wrong. `oc get tenants` is something else, still in TechPreview and not working.

When 3scale is installed with the operator/templates, a default tenant is created, with a "fixed" url:

`3scale-admin.${wildcardDomain}`

Being `wildcardDomain` one parameter provided during installation.

Documentation should tell the customer to head to that url to start working.

Optionally, the customer can create new tenants on the "MASTER portal URL"

`master.${wildcardDomain}`

The dashboard will show new tenant's portal url.

I think it is not necessary to document reading routes or quering tenant to get portal urls.

This issue goes for 2.6 and 2.7
This issue is also valid for installation using templates. No need to inspect routes as documented.