dmaengine: Fix spelling for parenthesis in dmatest documentation

Fix spelling for parenthesis in dmatest documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[ jc: did s/parenthesis/parentheses/ and reflowed ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>