Fix header formatting problem in wxMessageBox() documentation.

@header macro must be preceded by a new line.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70382 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/msgdlg.h

@@ -277,6 +277,7 @@ public:
         for @a x and @a y to let the system position the window.
     @param y
         Vertical dialog position (ignored under MSW).
+
     @header{wx/msgdlg.h}
 */
 int wxMessageBox(const wxString& message,