Fix link to MainLoop docs in README.md (#1491)

Link was broken with a 404.

README.md

@@ -51,7 +51,7 @@ In addition, a complete Xterm/Vt100 terminal emulator that you can embed is now
 * **[Flexible Layout](https://migueldeicaza.github.io/gui.cs/articles/overview.html#layout)** - **Terminal.Gui** supports both *Absolute layout* and an innovative UI layout system referred to as *Computed Layout*. *Computed Layout* makes it easy to layout controls relative to each other and enables dynamic console GUIs.
 * **Clipboard support** - Cut, Copy, and Paste of text provided through the [`Clipboard`](https://migueldeicaza.github.io/gui.cs/api/Terminal.Gui/Terminal.Gui.Clipboard.html) class.
 * **[Arbitrary Views](https://migueldeicaza.github.io/gui.cs/api/Terminal.Gui/Terminal.Gui.View.html)** - All visible UI elements are subclasses of the `View` class, and these in turn can contain an arbitrary number of sub-views.
-* **Advanced App Features** - The [Mainloop](https://migueldeicaza.github.io/gui.cs/api/Terminal.Gui/Mono.Terminal.MainLoop.html) supports processing events, idle handlers, timers, and monitoring file
+* **Advanced App Features** - The [Mainloop](https://migueldeicaza.github.io/gui.cs/api/Terminal.Gui/Terminal.Gui.MainLoop.html) supports processing events, idle handlers, timers, and monitoring file
 descriptors.
 * **Reactive Extensions Support** - Use [reactive extensions](https://github.com/dotnet/reactive) and benefit from increased code readability, and the ability to apply the MVVM pattern and [ReactiveUI](https://www.reactiveui.net/) data bindings. See the [source code](https://github.com/migueldeicaza/gui.cs/tree/master/ReactiveExample) of a sample app in order to learn how to achieve this.