[3538] Corrected the declaration of the example pool in the kea.conf

Recently the "pool" parameter has been renamed to "pools" and its type
was changed from the string to a map. The kea.conf was not updated to
use the new format for the pool definition. This change corrects it.

src/bin/keactrl/kea.conf.in

@@ -23,7 +23,7 @@
 # enable it.
   "subnet4": [
 #  {    "subnet": "192.0.2.0/24",
-#       "pool": [ "192.0.2.1 - 192.0.2.200" ] }
+#       "pools": [ { "pool": "192.0.2.1 - 192.0.2.200" } ] }
   ]
 },