Correct return value of CTaskDialog::OnHelp()

It returns `S_FALSE`, not `S_OK`, according to `afxtaskdialog.cpp` in 14.42.34433.

In testing, returning `S_OK` from an overridden implementation may result in another window (e.g., a main frame window) also presenting help.

Author: JoshHarmon

docs/mfc/reference/ctaskdialog-class.md

@@ -685,7 +685,7 @@ virtual HRESULT OnHelp();
 
 ### Return Value
 
-The default implementation returns S_OK.
+The default implementation returns S_FALSE.
 
 ### Remarks