Apply suggestion from @swissspidy

Author: swissspidy

features/config-get-field.feature

@@ -339,7 +339,8 @@ Feature: Get the value of a constant or variable defined in wp-config.php and wp
     When I run `wp config get WP_DEBUG --format=yaml`
     Then STDOUT should be:
       """
-      true
+      ---
+      - true
       """
 
     When I run `wp config get WP_DEBUG_LOG --format=yaml`