doc: usage: load: document part as hexadecimal

doc: usage: load: document part as hexadecimal

`part` option is in hexadecimal, so information is missing in usage
documentation.

Callgraph for `part` parsing is :
do_load -> fs_set_blk_dev -> part_get_info_by_dev_and_name_or_num ->
blk_get_device_part_str -> hextoul (This is why it is hexadecimal)

Signed-off-by: Mickaël Tansorier <mickael.tansorier@smile.fr>
Reviewed-by: Yoann Congal <yoann.congal@smile.fr>
Reviewed-by: Simon Glass <sjg@chromium.org>