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>