Update lease6-add.json

doc/api/lease6-add.json

 {
     "name": "lease6-add",
     "brief": "The lease6-add command allows for the creation of a new lease.",
-    "description": "See <xref linkend=\"lease4-add\"/>",
+    "description": "See <xref linkend=\"idp64\"/>",
     "support": [ "kea-dhcp4", "kea-dhcp6" ],
     "avail": "1.3.0",
     "hook": "lease_cmds",