fix: incorrect HTTP_PORT docker compose field in docker README.md

The override is [BACKEND_HTTP_PORT defined here](https://github.com/mempool/mempool/blob/59c513f2a52a96d125944b42230e1fb7b13b130e/docker/backend/start.sh#L7)

docker/README.md

@@ -124,7 +124,7 @@ Corresponding `docker-compose.yml` overrides:
     environment:
       MEMPOOL_NETWORK: ""
       MEMPOOL_BACKEND: ""
-      MEMPOOL_HTTP_PORT: ""
+      BACKEND_HTTP_PORT: ""
       MEMPOOL_SPAWN_CLUSTER_PROCS: ""
       MEMPOOL_API_URL_PREFIX: ""
       MEMPOOL_POLL_RATE_MS: ""