Fix build error in the example in readme.md

[cnwzhjs]

The example in the readme.md causes proc error:
error: proc macro panicked
--> build.rs:2:5
|
2 | / windows::build!(
3 | | Windows::Data::Xml::Dom::*,
4 | | Windows::Win32::System::WindowsProgramming::CloseHandle,
5 | | Windows::Win32::UI::WindowsAndMessaging::MessageBoxA,
... |
11 | | },
12 | | );
| |______^
|
= help: message: Could not find namespace Windows.Win32.System.WindowsProgramming

which is introduced by the namespace change.

This PR fixes this compile error

[ghost]

All CLA requirements met.

[MarijnS95]

I don't think this is correct, these are the old namespaces. Please see #782, all that needed to change is moving WaitForSingleObject under Threading (and a missing comma between the Threading::{} and SystemServices::{} block, that has been solved by reordering and not having SystemServices imports at all anymore).