[enterprise-4.2] Edit suggested in file installing/installing_restricted_networks/installing-restricted-networks-preparations.adoc

[kanekoh]

Which section(s) is the issue in?

Mirroring the OpenShift Container Platform image repository

What needs fixing?

The command mirroring the repository includes undefined environment variable 'UPSTREAM_REPO'.
According to the blog1, It seems not to be 'PRODUCT_REPO' but 'UPSTREAM_REPO'

[kalexand-rh]

It's an environment variable, so it just needs to be consistent. I got a comment during dev review that UPSTREAM_REPO made less sense in this context since we were directing users to the GA location. Thanks for pointing this out - they're all PRODUCT_REPO now.