Better variable substitution example

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docs/compose-file.md

@@ -1089,21 +1089,24 @@ It's more complicated if you're using particular configuration features:
 ## Variable substitution
 
 Your configuration options can contain environment variables. Compose uses the
-variable values from the shell environment in which `docker-compose` is run. For
-example, suppose the shell contains `POSTGRES_VERSION=9.3` and you supply this
-configuration:
+variable values from the shell environment in which `docker-compose` is run.
+For example, suppose the shell contains `EXTERNAL_PORT=8000` and you supply
+this configuration:
 
-    db:
-      image: "postgres:${POSTGRES_VERSION}"
+    web:
+      build: .
+      ports:
+        - "${EXTERNAL_PORT}:5000"
 
-When you run `docker-compose up` with this configuration, Compose looks for the
-`POSTGRES_VERSION` environment variable in the shell and substitutes its value
-in. For this example, Compose resolves the `image` to `postgres:9.3` before
-running the configuration.
+When you run `docker-compose up` with this configuration, Compose looks for
+the `EXTERNAL_PORT` environment variable in the shell and substitutes its
+value in. In this example, Compose resolves the port mapping to `"8000:5000"`
+before creating the `web` container.
 
 If an environment variable is not set, Compose substitutes with an empty
-string. In the example above, if `POSTGRES_VERSION` is not set, the value for
-the `image` option is `postgres:`.
+string. In the example above, if `EXTERNAL_PORT` is not set, the value for the
+port mapping is `:5000` (which is of course an invalid port mapping, and will
+result in an error when attempting to create the container).
 
 Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended shell-style
 features, such as `${VARIABLE-default}` and `${VARIABLE/foo/bar}`, are not