This unescaped backquote charcter confuses Sphinx, making it emit a
warning and slightly confused HTML in the end.
Let's escape it instead, to keep things sane.
Fixes: fdff24d930 ("docs: add release notes for 20.1.8")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6863>