message: doc: fix mistaken nested code block backticks
diff: @@ -281,15 +281,13 @@ After this, the indexer agent and service can be run as follows:
  2. Indexer Agent
  - ````sh
  + ```sh
  docker run -p 18000:8000 -it indexer-agent:latest ...
  ```
  This starts the indexer agent and serves the so-called indexer management API
  on the host at port 18000.
  - ````
  -
  ## Terraform & Kubernetes
  The [terraform/](./terraform/) and [k8s/](./k8s) directories provide a
  