Update CONFIGURATION.md to clarify that valid_status_codes expects a list (#1335)

The valid_status_codes expects a list between square brackets. Without you get this error: cannot unmarshal !!str `200, 401` into []int

Signed-off-by: Nikotine1 <[email protected]>

Author: Nikotine1

CONFIGURATION.md

@@ -47,8 +47,8 @@ modules:
 ### `<http_probe>`
 ```yml
 
-  # Accepted status codes for this probe. Defaults to 2xx.
-  [ valid_status_codes: <int>, ... | default = 2xx ]
+  # Accepted status codes for this probe. List between square brackets. Defaults to 2xx.
+  [ valid_status_codes: [<int>, ...] | default = 2xx ]
 
   # Accepted HTTP versions for this probe.
   [ valid_http_versions: <string>, ... ]