mtd: nand: typo in nand_id_has_period() comments

The simple example provided in the comments for nand_id_has_period()
actually has a period of 3, not 2. Silly mistake...

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>