Update cache-remove.json

doc/api/cache-remove.json

+An example command to remove an IPv4 host with reserved address 192.0.2.1 from subnet with a subnet-id 123 looks as follows:
 {
     "name": "cache-remove",
     "brief": "The cache-remove command works similarly to reservation-get command.",
@@ -12,6 +13,8 @@
         \"subnet-id\": 123
     }
 }
+
+Another example that removes IPv6 host identifier by DUID and specific subnet-id is:
 {
     \"command\": \"cache-remove\",
     \"arguments\": {