cmd_part: fix typo in part command help text

All the sub-commands start with the main command name, but it was
missing from one of the help texts.

<panto> typos fix.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>