[Serve] application level autoscaling doc missing import

[abrarsheikh]

What happened + What you expected to happen

policy example in application level autoscaling section is missing import for AutoscalingContext

Versions / Dependencies

master

Reproduction script

https://docs.ray.io/en/master/serve/advanced-guides/advanced-autoscaling.html#define-an-application-level-policy

Issue Severity

Low: It annoys or frustrates me.