[Doc][Serve] Import AutoscalingContext in autoscaling policy example (#57876)

## Description

## Related issues
Closes #57847

## Additional information

Signed-off-by: daiping8 <dai.ping88@zte.com.cn>

Author: daiping8

doc/source/serve/doc_code/autoscaling_policy.py

@@ -48,6 +48,7 @@ def custom_metrics_autoscaling_policy(
 # __begin_application_level_autoscaling_policy__
 from ray.serve._private.common import DeploymentID
 from typing import Dict, Tuple
+from ray.serve.config import AutoscalingContext
 
 
 def coordinated_scaling_policy(