ls-tree: fix documentation of %x format placeholder

ls-tree: fix documentation of %x format placeholder

ls-tree --format expands %x followed by two hexadecimal digits to the
character indicated by that hexadecimal number, e.g.:

   $ git ls-tree --format=%x41 HEAD | head -1
   A

It rejects % followed by a hexadecimal digit, e.g.:

   $ git ls-tree --format=%41 HEAD | head -1
   fatal: bad ls-tree format: element '41' does not start with '('

This functionality is provided by strbuf_expand_literal_cb(), which has
not been changed since it was factored out by fd2015b323 (strbuf:
separate callback for strbuf_expand:ing literals, 2019-01-28).

Adjust the documentation accordingly.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>