[fix, UX] naturallightwidget: Reduce text size in button

'Save settings' was too large a string for larger fonts, so replace
with just 'Save' (issue #3825).

frontend/ui/widget/naturallightwidget.lua

@@ -296,7 +296,7 @@ function NaturalLightWidget:createMainContent(width, height)
         end,
     }
     local button_ok = Button:new{
-        text = "Save settings",
+        text = "Save",
         margin = Size.margin.small,
         radius = 0,
         width = self.width * 0.2,