Doc: z8530book: Fix typo in API-z8530-sync-txdma-open.html

This patch fix a spelling typo found in API-z8530-sync-txdma-open.html.
It is because this file was generated from comment in source,
I have to fix comment in source.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>