Change a typo'd MIME type in a comment. The Content-Type is
application/xhtml+xml, not application/xhtm+xml.
Fixes up code originally added in 53c4031 ("gitweb: Strip
non-printable characters from syntax highlighter output", 2011-09-16).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Acked-by: Jakub Narębski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>