modules/understanding-upgrade-channels: Fix '{product-stable}' -> '{product-version}' typo
modules/understanding-upgrade-channels: Fix '{product-stable}' -> '{product-version}' typo

Typo is from 1c199f19e0 (Mod channel doc, 2020-02-07, #19088).

vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs,vikram-redhat/openshift-docs