Fix missing --ipv6 flag in IPVLAN L3 dual-stack example

The L3 dual-stack IPVLAN example omitted the --ipv6 flag when creating
the network, causing Docker to default to EnableIPv6: false despite an
IPv6 subnet being specified. The L2 example already includes this flag.

Fixes #19337

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dvdksn

content/manuals/engine/network/drivers/ipvlan.md

@@ -534,7 +534,7 @@ in order to forward broadcast and multicast packets.
 $ docker network create -d ipvlan \
     --subnet=192.168.110.0/24 \
     --subnet=192.168.112.0/24 \
-    --subnet=2001:db8:abc6::/64 \
+    --ipv6 --subnet=2001:db8:abc6::/64 \
     -o parent=eth0 \
     -o ipvlan_mode=l3 ipnet110