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>
diff --git a/doc/usage/cmd/load.rst b/doc/usage/cmd/load.rst
index 1efee77..2c892ee 100644
--- a/doc/usage/cmd/load.rst
+++ b/doc/usage/cmd/load.rst
@@ -40,7 +40,7 @@
 pos
     number of bytes to skip
 
-addr, bytes, pos are hexadecimal numbers.
+part, addr, bytes, pos are hexadecimal numbers.
 
 Example
 -------