Documentation example starlark script

The script does not compile

https://docs.drone.io/pipeline/scripting/starlark/

should be:

def main(ctx):
  return {
    "kind": "pipeline",
    "name": "build",
    "steps": [
      {
        "name": "build",
        "image": "alpine",
        "commands": [
            "echo hello world",
        ],
      },
    ],
  }

(would do PR but not sure where docs are…)