$ prefect -v
2.0b7

$ prefect orion kubernetes-manifest
Error: No such command 'kubernetes-manifest'.

$ prefect orion --help
Commands:
  database  Commands for interacting with the database.
  start     Start an Orion server

The documents needs to be updated? https://orion-docs.prefect.io/tutorials/kubernetes-flow-runner/

Yes it does. See this