docs: trace: ring-buffer-design.rst: use the new SPDX tag

SPDX v3.10 gained support for GFDL-1.2 with no invariant sections:

	https://spdx.org/licenses/GFDL-1.2-no-invariants-only.html



Let's use it, instead of keeping a license text for this file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/dbc9bd9ab30c6862e465343239e82102cbdc0f39.1599628249.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>