Correct the example in composer/Plugin/Scaffold/README.md

Problem/Motivation

Follow-up of #2982684: Add a composer scaffolding plugin to core.

The example in composer/Plugin/Scaffold/README.md contains minor bugs.

Proposed resolution

1. Change drupal/composer-scaffold to drupal/core-composer-scaffold.

 {
   "name": "my/project",
   "require": {
-    "drupal/composer-scaffold": "*",
+    "drupal/core-composer-scaffold": "*",
     "composer/installers": "^1.2",
     "cweagans/composer-patches": "^1.6.5",

2. Delete trailing comma, because this is not valid JSON.

   "extra": {
     "composer-scaffold": {
       "allowed-packages": [
-        "drupal/core",
+        "drupal/core"
       ],

Remaining tasks

Add credits for greg.1.anderson and mohit_aghera. They added notes to the documentation Using Drupal's Composer Scaffold. See the discussions.