Clarify the format for file SD files.

This example has confused many potential users who
didn't realise it needed to be put in a list.

content/docs/operating/configuration.md

@@ -472,15 +472,18 @@ all defined files are detected via disk watches and applied immediately. Files m
 provided in YAML or JSON format. Only changes resulting in well-formed target groups
 are applied.
 
-The JSON version of a target group has the following format:
+The JSON file must contain a list of target groups, using this format:
 
 ```
-{
+[
+  {
     "targets": [ "<host>", ... ],
     "labels": {
       [ "<labelname>": "<labelvalue>", ... ]
     }
-}
+  },
+  ...
+]
 ```
 
 As a fallback, the file contents are also re-read periodically at the specified