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>

Documentation/git-ls-tree.txt

@@ -86,9 +86,9 @@ OPTIONS
 --format=<format>::
 	A string that interpolates `%(fieldname)` from the result
 	being shown. It also interpolates `%%` to `%`, and
-	`%xx` where `xx` are hex digits interpolates to character
-	with hex code `xx`; for example `%00` interpolates to
-	`\0` (NUL), `%09` to `\t` (TAB) and `%0a` to `\n` (LF).
+	`%xNN` where `NN` are hex digits interpolates to character
+	with hex code `NN`; for example `%x00` interpolates to
+	`\0` (NUL), `%x09` to `\t` (TAB) and `%x0a` to `\n` (LF).
 	When specified, `--format` cannot be combined with other
 	format-altering options, including `--long`, `--name-only`
 	and `--object-only`.