Correct Default theme.json (#1872)

The theme.json file uses "parent_theme" instead of "extends", which doesn't match the documentation. This has been corrected for a case where the user may copy and paste the default theme's theme.json file and expects it to work.

resources/views/layouts/default/theme.json

@@ -1,4 +1,4 @@
 {
   "name": "default",
-  "parent_theme": null
+  "extends": null
 }