Add credit for beltzner for current book of mozilla text. b=411352, r=gavin, a=mconnor

toolkit/content/mozilla.xhtml

@@ -31,6 +31,7 @@
    -   Henrik Gemal <mozilla@gemal.dk>
    -   Alexey Chernyak <alexeyc@bigfoot.com> (XHTML 1.1 conversion)
    -   Johnathan Nightingale <johnath@mozilla.com>
+   -   Mike Beltzner <beltzner@mozilla.com>
    -
    - Alternatively, the contents of this file may be used under the terms of
    - either the GNU General Public License Version 2 or later (the "GPL"), or