updating/updating-restricted-network-cluster: Add "signature" to heading
updating/updating-restricted-network-cluster: Add "signature" to heading

The ConfigMap contains a signature (or signatures) for the release
image, not an image itself.  This also matches a number of existing
references:

  $ git --no-pager grep -i 'image signature configmap' modules
  modules/update-configuring-image-signature.adoc:= Creating an image signature ConfigMap manually
  modules/update-configuring-image-signature.adoc:Create and apply the image signature ConfigMap to the cluster that you want to update.
  modules/update-oc-configmap-signature-verification.adoc:. Apply the the mirrored release image signature ConfigMap to the connected cluster:
  modules/update-restricted.adoc:* You applied the release image signature ConfigMap for the new release to your cluster.
  modules/update-restricted.adoc:* You obtained the sha256 sum value for the release from the image signature ConfigMap.
  modules/update-restricted.adoc:<1> The `<sha256_sum_value>` value is the sha256 sum value for the release from the image signature ConfigMap, for example, `@sha256:81154f5c03294534e1eaf0319bef7a601134f891689ccede5d705ef659aa8c92`

I'm not sure if there's a way to preserve the previous anchor for
folks who may have existing links, so I'm just changing the anchor to
keep up with the header here.

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