Documentation/DMA-API-HOWTO.txt: fix typo

Noted by Jesper

Cc: Jesper Juhl <jj@chaosbits.net>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>