docs: coresight: fix `make htmldocs` warning

Fix `make htmldocs` warning:

Documentation/trace/coresight/coresight-ect.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: https://lore.kernel.org/r/20200430203612.90404-1-vitor@massaru.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/coresight/coresight-ect.rst

 .. SPDX-License-Identifier: GPL-2.0
+
 =============================================
 CoreSight Embedded Cross Trigger (CTI & CTM).
 =============================================