Examples: fix superfluous ';' in undo example

There was a superfluous ';' at the end of main. No compiler will
complain I guess but the examples should be syntactically correct.

Task-number: QTBUG-55128
Change-Id: I2ddc3041ed05106d04fb4f4ad74735c72c9ddb26
Reviewed-by: André Hartmann <aha_1980@gmx.de>

examples/widgets/tools/undo/main.cpp

@@ -62,4 +62,4 @@ int main(int argc, char **argv)
     win.show();
 
     return app.exec();
-};
+}