Doc: Change the wording a bit for the HOSTNAME environment variable

I should have done this while merging #9474.

docs/reference/setup/configuration.asciidoc

@@ -208,10 +208,10 @@ node:
   name: <NAME OF YOUR NODE>
 --------------------------------------------------
 
-The hostname of the machine is provided in an
-environment variable, so you can set the node name
-to the hostname, if on that machine you only run _a
-single elasticsearch node_ for that cluster.
+The hostname of the machine is provided in the environment
+variable `HOSTNAME`. If on your machine you only run a
+single elasticsearch node for that cluster, you can set
+the node name to the hostname using the `${...}` notation:
 
 [source,yaml]
 --------------------------------------------------