Documentation: dpaa2: Use correct heading adornment

Add overline heading adornment to document title in order to comply
with kernel doc requirements.

Fixes: 60b91319 staging: fsl-mc: Convert documentation to rst format
Signed-off-by: NIoana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/dpaa2/overview.rst

 .. include:: <isonum.txt>
 
+=========================================================
 DPAA2 (Data Path Acceleration Architecture Gen2) Overview
 =========================================================