docs(dashboard): fix missing syntax highlight (#12910)

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

docs/usage/key-concepts/dashboard.md

@@ -29,7 +29,7 @@ To turn on the Dashboard manually, add the `:dependencyDashboard` preset to your
 
 Or set `dependencyDashboard` to `true`:
 
-```
+```json
 {
   "dependencyDashboard": true
 }