m4: Fix typo in usage comment
* gettext-runtime/m4/gettext.m4: Remove duplicate words.
* gettext-runtime/m4/intl.m4: Likewise.
* gettext-runtime/m4/intldir.m4: Likewise.
* gettext-runtime/m4/intlmacosx.m4: Likewise.
* gettext-runtime/m4/lcmessage.m4: Likewise.
* gettext-runtime/m4/nls.m4: Likewise.
* gettext-runtime/m4/po.m4: Likewise.
* gettext-runtime/m4/progtest.m4: Likewise.
Reported by Jakub Wilk in:
https://lists.gnu.org/archive/html/bug-gettext/2015-10/msg00026.html.
