docs: trace: coresight-ect.rst: Fix a build warning

Sphinx wants a line after "..", as otherwise it complains with:

	Documentation/trace/coresight/coresight-ect.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200518180242.7916-8-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/trace/coresight/coresight-ect.rst

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