mtd: nand: Fix nanddev_pos_next_page() kernel-doc header

Function name is wrong in the kernel-doc header.

Fixes: 9c3736a3

 ("mtd: nand: Add core infrastructure to deal with NAND devices")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>