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.

src/configs/tools.json

@@ -210,7 +210,7 @@
     "type": "custom-function",
     "repository": "sebastianbergmann/phpunit",
     "domain": "https://phar.phpunit.de",
-    "function": "phpcpd",
+    "function": "phpunit",
     "version_prefix": "",
     "version_parameter": "--version"
   },