docs/about/license: Update LICENSE URL

In early 2021 (see commit 2ad7843 "docs: update README to use
GitLab repo URLs") almost all of the code base was converted to
point to GitLab instead of git.qemu.org. During 2023, git.qemu.org
switched from a git mirror to a http redirect to GitLab (see [1]).

Update the LICENSE URL to match its previous content, displaying
the file raw content similarly to gitweb 'blob_plain' format ([2]).

[1] https://lore.kernel.org/qemu-devel/CABgObfZu3mFc8tM20K-yXdt7F-7eV-uKZN4sKDarSeu7DYoRbA@mail.gmail.com/
[2] https://git-scm.com/docs/gitweb#Documentation/gitweb.txt-blobplain

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230822125716.55295-1-philmd@linaro.org>

docs/about/license.rst

@@ -8,4 +8,4 @@ QEMU is a trademark of Fabrice Bellard.
 QEMU is released under the `GNU General Public
 License <https://www.gnu.org/licenses/gpl-2.0.txt>`__, version 2. Parts
 of QEMU have specific licenses, see file
-`LICENSE <https://git.qemu.org/?p=qemu.git;a=blob_plain;f=LICENSE>`__.
+`LICENSE <https://gitlab.com/qemu-project/qemu/-/raw/master/LICENSE>`__.