docs: Fix IPv6 example for Compose (#3531)

The subnet must be specified as part of `ipam.configs`.

This was unfortunately slightly incorrect due to a mistake in the official Docker docs being propagated, which has since been fixed upstream.

Refer to the official Compose Spec for more details:
 * https://docs.docker.com/compose/compose-file/06-networks/#ipam

docs/content/config/advanced/ipv6.md

@@ -92,7 +92,9 @@ Next, configure a network with an IPv6 subnet for your container with any of the
             networks:
               dms-ipv6:
                 enable_ipv6: true
-                subnet: fd00:cafe:face:feed::/64
+                ipam:
+                  config:
+                    - subnet: fd00:cafe:face:feed::/64
             ```
 
             ??? tip "Override the implicit `default` network"