Fix mistake in #[Sealed] example.

Author: ZebulanStanphill

README.md

@@ -220,7 +220,7 @@ E.g.
 
 ```php
 
-#[Sealed([Success::class, Failure::class])]
+#[Sealed(Success::class, Failure::class)]
 abstract class Result {} // Result can only be extended by Success or Failure
 
 // OK