software/headless-chromium: fix software schema json

software/headless-chromium/software.cfg.json

 {
   "name": "Headless Chromium",
   "description": "Headless (stripped-down) Chromium shell",
-  "serialization": "xml",
+  "serialisation": "xml",
   "software-type": {
     "default": {
       "title": "Default",