Documentation: networking: dsa: add missing new line in devlink section

"make htmldocs" produces these warnings:
Documentation/networking/dsa/dsa.rst:468: WARNING: Unexpected indentation.
Documentation/networking/dsa/dsa.rst:477: WARNING: Block quote ends without a blank line; unexpected unindent.

Fixes: 8411abbc ("Documentation: networking: dsa: mention integration with devlink")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>