API gateway guide: revert to inbound (#1002)

I didn't realize that the K8s Operator doesn't yet support phase-based
naming, and while it's coming soon
(ngrok/ngrok-operator#456), I'd rather this work
correctly right now and update it again in the future.

docs/guides/api-gateway/kubernetes.mdx

@@ -286,7 +286,7 @@ metadata:
   namespace: prod
 spec:
   policy:
-    on_http_request:
+    inbound:
       - name: "Rate limit POST requests"
         expressions:
           - "req.method == 'POST' || req.method == 'PUT'"