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

Documentation/trace/ring-buffer-design.rst

-.. This file is dual-licensed: you can use it either under the terms
-.. of the GPL 2.0 or the GFDL 1.2 license, at your option. Note that this
-.. dual licensing only applies to this file, and not this project as a
-.. whole.
-..
-.. a) This file is free software; you can redistribute it and/or
-..    modify it under the terms of the GNU General Public License as
-..    published by the Free Software Foundation version 2 of
-..    the License.
-..
-..    This file is distributed in the hope that it will be useful,
-..    but WITHOUT ANY WARRANTY; without even the implied warranty of
-..    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-..    GNU General Public License for more details.
-..
-.. Or, alternatively,
-..
-.. b) Permission is granted to copy, distribute and/or modify this
-..    document under the terms of the GNU Free Documentation License,
-..    Version 1.2 version published by the Free Software
-..    Foundation, with no Invariant Sections, no Front-Cover Texts
-..    and no Back-Cover Texts. A copy of the license is included at
-..    Documentation/userspace-api/media/fdl-appendix.rst.
-..
-.. TODO: replace it to GPL-2.0 OR GFDL-1.2 WITH no-invariant-sections
+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.2-no-invariants-only
 
 ===========================
 Lockless Ring Buffer Design