drm/panic: add missing Markdown code span

Add missing Markdown code span.

This was found using the Clippy `doc_markdown` lint, which we may want
to enable.

Fixes: cb5164ac ("drm/panic: Add a QR code panic screen")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250324210359.1199574-3-ojeda@kernel.org