Change LPPAINTSTRUCT to *const PAINTSTRUCT (#32)

Author: hugocowan

book_src/opening_a_window/win32.md

@@ -1374,7 +1374,7 @@ Sounds easy enough to do.
 #[link(name = "User32")]
 extern "system" {
   /// [`BeginPaint`](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-beginpaint)
-  pub fn BeginPaint(hWnd: HWND, lpPaint: LPPAINTSTRUCT) -> HDC;
+  pub fn BeginPaint(hWnd: HWND, lpPaint: *const PAINTSTRUCT) -> HDC;
 
   /// [`FillRect`](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-fillrect)
   pub fn FillRect(hDC: HDC, lprc: *const RECT, hbr: HBRUSH) -> c_int;