Fix getting-started.md so that `composer.json` contains valid JSON

The suggested content for `composer.json` is invalid JSON and causes the following error if used as is:

  [Seld\JsonLint\ParsingException]
  "composer.json" does not contain valid JSON
  Parse error on line 3:
}
  ----------------------^
  Expected: 'STRING' - It appears you have an extra trailing comma