Fix typo in ISO url

Author: khos2ow

tools/appliance/builtin/template.json

@@ -23,7 +23,7 @@
       "disk_interface": "virtio",
       "net_device": "virtio-net",
 
-      "iso_url": "http://mirror.nbrc.ac.in/centos/7/isos/x86_64/CentOS-7-x86_64-Minimal-1808.iso",
+      "iso_url": "http://mirror.nbrc.ac.in/centos/7/isos/x86_64/CentOS-7-x86_64-Minimal-1804.iso",
       "iso_checksum": "13675c6f74880e7ff3481b91bdaf925ce81bda8f",
       "iso_checksum_type": "md5",