doc: sandbox: Fix the "sb" command name

doc: sandbox: Fix the "sb" command name

The command name was "sbi" instead of "sb" in "doc/usage/cmd/sb.rst",
the file documenting the "sb" command. It is annoying, because the
index in the left panel on the
<https://docs.u-boot.org/en/latest/usage/cmd/sb.html> page shows no
"sb" command, which makes difficult to navigate to the "sb"
documentation.

Fixed the command name: "sbi" -> "sb".

Fixes: ec6d30649cd5 (doc: sandbox: Add docs for the sb command, 2024-10-28)
Signed-off-by: Olivier L'Heureux <olivier.lheureux@mind.be>
Reviewed-by: Simon Glass <sjg@chromium.org>