Documentation: correct parameter error for dma_mapping_error

dma_mapping_error takes two parameters, but some of examples
in Documentation/DMA-API-HOWTO.txt just takes one. So correct
it.

Signed-off-by: Liu Hua <sdu.liu@huawei.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>