update doc

Author: ogios

data/doc/4.widgets/4.backlight.md

@@ -34,7 +34,7 @@ const `backlight`
 
 ## `device_name`
 
-Hardware name, you can find it under `/sys/class/backlight.
+Hardware name, you can find it under `/sys/class/backlight`.
 
 Default: `empty` to use default from `org.freedesktop.login1.Session`