docs(config): correct the app.debug help text

In the example `config/config.dist.php` file correct the help text about
the `app.debug` setting.

Author: JohnVillalovos

config/config.dist.php

@@ -19,7 +19,8 @@
         # The public name of the application
         'app.title' => 'LibreBooking',
 
-        # The public name of the application
+        # Enable or disable debug mode for the application
+        # if enabled it will enable 'display_errors' and 'display_startup_errors'
         'app.debug' => false,
 
         # Administrator email address