docs: Use HTML comments for the kernel-toc SPDX line

This line was initially placed in {# jinja2 comments #}, but that led to an
"invalid token" complaint from spdxcheck.py.  Rather than fix the script
for a usage we'll likely never see anywhere else, just switch to an HTML
comment, which spdxcheck.py thinks is fine.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/templates/kernel-toc.html

-{# SPDX-License-Identifier: GPL-2.0 #}
+<!-- SPDX-License-Identifier: GPL-2.0 -->
 {# Create a local TOC the kernel way #}
 <p>
 <h3 class="kernel-toc-contents">Contents</h3>