Fix broken link (#354)

Fixing the TopLevel link

docfx/articles/mainloop.md

@@ -30,7 +30,7 @@ provides access to these functions.
 
 When your code invokes `Application.Run (Toplevel)`, the application
 will prepare the current
-`Toplevel`[../api/Terminal.Gui/Terminal.Gui.Toplevel.html] instance by
+[`Toplevel`](../api/Terminal.Gui/Terminal.Gui.Toplevel.html) instance by
 redrawing the screen appropriately and then calling the mainloop to
 run.    
 
@@ -158,4 +158,4 @@ written without blocking the application.
 To do this, you on Unix, you can cast the `MainLoop` instance to a
 [`UnixMainLoop`](docs/api/Mono.Terminal/Mono.Terminal.UnixMainLoop.html)
 and use the `AddWatch` method to register an interest on a particular
-condition.
+condition.