Fix a broken internal link

The fix of 60f51b3 is inadequate.
(Somehow my local environment it worked, but it doesn't work on
the production. It is because `.` is not removed from DOM-id on
the production environment.)

Signed-off-by: Daijiro Fukuda <[email protected]>

Author: daipom

configuration/config-file.md

@@ -507,7 +507,7 @@ These parameters are reserved and are prefixed with an `@` symbol:
 
 * `@label`: specifies the label symbol. See
 
-  [label](config-file.md#5-group-filter-and-output-the-label-directive)
+  [label](config-file.md#5.-group-filter-and-output-the-label-directive)
 
   section.