Plug.vala: more concise placeholder copy (#81)

Co-authored-by: Danielle Foré <danielle@elementary.io>

src/Plug.vala

@@ -25,7 +25,7 @@ public class NotificationsPlug : Switchboard.Plug {
 
         var description = _("Notifications preferences are for configuring which apps make use of notifications, for changing how an app's notifications appear,\nand for setting when you do not want to be disturbed by notifications.");
         description += "\n\n";
-        description += _("When apps are installed that have notification options they will automatically appear here.");
+        description += _("Apps with configurable notifications will appear here once installed.");
 
         var icon_name = "dialog-information";