remove incorrect rootFSUrl field

Author: aireilly

modules/ztp-configuring-the-cluster-for-a-disconnected-environment.adoc

@@ -85,10 +85,9 @@ spec:
     name: 'assisted-installer-mirror-config'
   osImages:
     - openshiftVersion: <ocp_version>
-      rootfs: <rootfs_url> <1>
       url: <iso_url> <1>
 ----
-<1> Must match the URLs of the HTTPD server.
+<1> Must match the URL of the HTTPD server.
 
 [IMPORTANT]
 ====