WorldProviderManager: removed incorrect param doc

this was used to trick phpstorm, but PHPStan doesn't like it, and it isn't consistently used anyway.

src/world/format/io/WorldProviderManager.php

@@ -47,7 +47,7 @@ abstract class WorldProviderManager{
 	/**
 	 * Returns the default format used to generate new worlds.
 	 *
-	 * @return string|WritableWorldProvider
+	 * @return string
 	 */
 	public static function getDefault() : string{
 		return self::$default;