docs: networking: dpaa2: fix chapter title format

Fix the DPAA2 DPIO driver chapter title by adding the necessary
overline. Without this, the index page of the DPAA2 documentation
doesn't display properly.

Fixes: d8e516ba ("soc: fsl: dpio: Convert DPIO documentation to .rst")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20210722100356.635078-5-ciorneiioana@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/networking/device_drivers/ethernet/freescale/dpaa2/dpio-driver.rst

 .. include:: <isonum.txt>
 
+===================================
 DPAA2 DPIO (Data Path I/O) Overview
 ===================================