[uid] Fix Uuid::v8() usage example to show it requires a UUID string

This update corrects the usage example of Uuid::v8() to prevent misuse and confusion

Author: Hmache

components/uid.rst

@@ -140,7 +140,7 @@ of the UUID value is specific to each implementation and no format should be ass
     use Symfony\Component\Uid\Uuid;
 
     // $uuid is an instance of Symfony\Component\Uid\UuidV8
-    $uuid = Uuid::v8();
+    $uuid = Uuid::v8('d9e7a184-5d5b-11ea-a62a-3499710062d0');
 
 If your UUID value is already generated in another format, use any of the
 following methods to create a ``Uuid`` object from it::