[xamarin] fix Mono runtime version detection (#1429)

Context: https://github.com/mono/mono/blob/0a37eeadece5c2cec8a6e1f43a94f0b323a22513/mcs/class/corlib/Mono/Runtime.cs#L68-L73

`Mono.Runtime.GetDisplayName` is `public` on Xamarin platforms, and so
we need to include `BindingFlags.Public` to call it via reflection.

I updated the screenshot in the documentation to reflect what the
value looks like running on Xamarin.Android in VS 2019 16.5.

I also took an attempt at fixing the documentation for Xamarin, I
think the link is just missing. Hopefully this URL will work now:

    https://benchmarkdotnet.org/articles/samples/IntroXamarin.html

Author: jonathanpeppers

docs/articles/samples/toc.yml

@@ -111,4 +111,6 @@
 - name: IntroTailcall
   href: IntroTailcall.md
 - name: IntroUnicode
-  href: IntroUnicode.md
+  href: IntroUnicode.md
+- name: IntroXamarin
+  href: IntroXamarin.md

docs/images/xamarin-screenshot.png

@@ -121,7 +121,7 @@ internal static string GetRuntimeVersion()
             if (IsMono)
             {
                 var monoRuntimeType = Type.GetType("Mono.Runtime");
-                var monoDisplayName = monoRuntimeType?.GetMethod("GetDisplayName", BindingFlags.NonPublic | BindingFlags.Static);
+                var monoDisplayName = monoRuntimeType?.GetMethod("GetDisplayName", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static);
                 if (monoDisplayName != null)
                 {
                     string version = monoDisplayName.Invoke(null, null)?.ToString();