Fixed Typo Error on Platform documentation

I fixed a typographical error in the documentation. The static method "ofSystem" does not exist on the Symfony\AI\Platform\Message class, it should be "forSystem"

Author: Horlerdipo

src/platform/doc/index.rst

@@ -122,7 +122,7 @@ have different content types, like ``Text``, ``Image`` or ``Audio``, and can be
 
     // Create a message bag with a user message
     $messageBag = new MessageBag(
-        Message::ofSystem('You are a helpful assistant.')
+        Message::forSystem('You are a helpful assistant.')
         Message::ofUser('Please describe this picture?', Image::fromFile('/path/to/image.jpg')),
     );