mtd: cmdlinepart: Update comment for introduction of OFFSET_CONTINUOUS

The comment about offset zero was not updated when changing behavior:
  - Automatic offset calculation is indicated by OFFSET_CONTINUOUS,
  - Zero really means offset zero.

Fixes: b175d03d ("[PATCH] mtd cmdlinepart: allow zero offset value")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>