message: Tools: fix typo
  While the PHPCPD and PHPUnit tools use the [same function under the hood](https://github.com/shivammathur/setup-php/blob/1fa3ba1b162e818eed12eed233c9837299574948/src/tools.ts#L461-L473), I have the impression that the `function` keyword should be the tool name.
  If that's correct, this should fix it.
diff: "type": "custom-function",
  "repository": "sebastianbergmann/phpunit",
  "domain": "https://phar.phpunit.de",
  - "function": "phpcpd",
  + "function": "phpunit",
  "version_prefix": "",
  "version_parameter": "--version"
  },
  