If 'rd_nr' param was not specified, 16 (can be adjusted via
CONFIG_BLK_DEV_RAM_COUNT) devices would be created by default
but comment said 1. Fix it.
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>