CNV-12193: updating HCO variable for 2.6.4

Author: ousleyp

modules/virt-document-attributes.adoc

@@ -14,7 +14,7 @@
 :ProductVersion:
 :VirtVersion: 2.6
 :KubeVirtVersion: v0.36.2
-:HCOVersion: 2.6.3
+:HCOVersion: 2.6.4
 // :LastHCOVersion:
 :product-build:
 :DownloadURL: registry.access.redhat.com