dmaengine: doc : Fix warning "Title underline too short" while make xmldocs

This patch fix following warning during 'make xmldocs'

Documentation/driver-api/dmaengine/client.rst:188:
WARNING: Title underline too short.

Further APIs:
------------
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dmaengine/client.rst

@@ -185,7 +185,7 @@ The details of these operations are:
       void dma_async_issue_pending(struct dma_chan *chan);
 
 Further APIs:
-------------
+-------------
 
 1. Terminate APIs