Documentation: fix Sphinx warning in xilinx_sdfec.rst

Fix Sphinx format warning by adding a blank line.

Documentation/misc-devices/xilinx_sdfec.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Dragan Cvetic <dragan.cvetic@xilinx.com>
Link: https://lore.kernel.org/r/8d644cf1-fa7b-ec62-84cf-9b41d7c30eed@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/misc-devices/xilinx_sdfec.rst

 .. SPDX-License-Identifier: GPL-2.0+
+
 ====================
 Xilinx SD-FEC Driver
 ====================