dt-bindings: mtd: partitions: u-boot: Fix typo

The initial description contained a typo.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231218130656.9020-1-wahrenst@gmx.net