doc: dfu: describe eMMC partition number parameter

In dfu_alt_info for eMMC the eMMC partition number can be specified.

The separator in dfu_alt_info is a semicolon not a comma.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Lukasz Majewski <lukma@denx.de>