Update offline.yml (#8358)

[cni-plugins] upgrade to stable 1.0.1 (#8331) using flannel cni add flannel_cni_download_url

flannel_cni_download_url offline doc update

inventory/sample/group_vars/all/offline.yml

@@ -36,6 +36,9 @@
 # [Optional] Calico with kdd: If using Calico network plugin with kdd datastore
 # calico_crds_download_url: "{{ files_repo }}/kubernetes/calico/{{ calico_version }}.tar.gz"
 
+# [Optional] Flannel: If using Falnnel network plugin
+# flannel_cni_download_url: "{{ files_repo }}/kubernetes/flannel/{{ flannel_cni_version }}/flannel-{{ image_arch }}"
+
 # [Optional] helm: only if you set helm_enabled: true
 # helm_download_url: "{{ files_repo }}/helm-{{ helm_version }}-linux-{{ image_arch }}.tar.gz"