doc: fix hex code escapes in git-ls-files

doc: fix hex code escapes in git-ls-files

The --format option on the git-ls-files man page states that `%xx`
interpolates to the character with hex code `xx`. This mirrors the
documentation and behavior of `git for-each-ref --format=...`. However,
in reality it requires the character with code `XX` to be specified as
`%xXX`, mirroring the behaviour of  `git log --format`.

Signed-off-by: Jayson Rhynas <jayrhynas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>