Update nf-winuser-getmenuitemcount.md

[iPhilip]

It's not clear why the hMenu parameter is optional.

[prmerger-automator]

@iPhilip : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[jwmsft]

@iPhilip, thanks for your PR submissions.
Parameters are auto generated from the header file. WinUser.h defines hMenu as optional (although I'm not sure why it does).
From WinUser.h:

int
WINAPI
GetMenuItemCount(
    _In_opt_ HMENU hMenu);

The current doc seems to be correct, so I will close this PR.