libfdt: fix description of fdt_get_string()

Looks like this comment was copied from that of
fdt_get_string_index().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: 5094eb40 ("fdt: Add functions to retrieve strings")
Acked-by: Simon Glass <sjg@chromium.org>