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>