Extend the example in the committers guide with the SPDX license identifier.

This should encourage people to use these identifiers to highlight our
licensing advantages by adopting SPDX tags.

Submitted by:	pfg@

en_US.ISO8859-1/articles/committers-guide/article.xml

@@ -2737,6 +2737,8 @@ Relnotes:           yes</programlisting>
       text as the preferred license scheme:</para>
 
     <programlisting>/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) [year] [your name]
  * All rights reserved.
  *