Fix unused Self import in example code (pylint violation)

The string annotation '"Self"' caused pylint to flag 'Self' as an
unused import since string annotations are not resolved at import time.
Remove the quotes so Self is referenced directly as a type annotation,
which satisfies pylint.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/dispatch/_actor_dispatcher.py

@@ -109,7 +109,7 @@ class ActorDispatcher(BackgroundService):
         ```python
         import os
         import asyncio
-        from typing import Any
+        from typing import Any, Self
         from typing import override
         from frequenz.dispatch import Dispatcher, ActorDispatcher, DispatchInfo
         from frequenz.client.common.microgrid.components import ComponentCategory
@@ -134,7 +134,7 @@ def new_with_dispatch(
                     dispatch_updates_receiver: Receiver[DispatchInfo],
                     *,
                     name: str | None = None,
-            ) -> "MyActor":
+            ) -> Self:
                 self = cls(name=name)
                 self._dispatch_updates_receiver = dispatch_updates_receiver
                 self._update_dispatch_information(initial_dispatch)