docs: corrected that entry in the specs (#1489)

aligned extensions to be fileExtensions

Author: mfranzke

packages/docs/src/docs/advanced-config-options.md

@@ -198,7 +198,7 @@ Other engine specific configuration options can be added and will be passed to t
   "engines": {
     "handlebars": {
       "package": "@pattern-lab/engine-handlebars",
-      "extensions": [
+      "fileExtensions": [
         "handlebars",
         "hbs"
       ],