Update the Spectre.Console.Cli documentation with CancellationToken

Now that #1911 is merged, all `Execute` methods of commands have a new CancellationToken parameter. Update the documentation to reflect that.

New documentation shall still be written with an example on how to pass the top-level CancellationToken to the app.Run(Async) method.

docs/input/cli/commands.md

@@ -18,7 +18,7 @@ public class HelloCommand : Command<HelloCommand.Settings>
     }
 
 
-    public override int Execute(CommandContext context, Settings settings)
+    public override int Execute(CommandContext context, Settings settings, CancellationToken cancellationToken)
     {
         AnsiConsole.MarkupLine($"Hello, [blue]{settings.Name}[/]");
         return 0;