mtd: rawnand: Fix a couple of spelling mistakes in Kconfig

There are two spelling mistakes in the Kconfig text. Fix them.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210704094705.37175-1-colin.king@canonical.com