Change license of .pro file

According to QUIP-18 [1], all build system files should be
BSD-3-Clause

[1]: https://contribute.qt-project.org/quips/18

Pick-to: 6.7
Change-Id: Ifd2832708e0c8e0bde1cad4004839fecffe44e3c
Reviewed-by: Kai Köhne <kai.koehne@qt.io>

qmake/doc/snippets/code/doc_src_qmake-manual.pro

@@ -1,5 +1,5 @@
 // Copyright (C) 2016 The Qt Company Ltd.
-// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
+// SPDX-License-Identifier: BSD-3-Clause
 
 #! [0]
 make all