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: cb5164ac43d0 ("drm/panic: Add a QR code panic screen")
Signed-off-by: Miguel Ojeda <[e-mail]>
Reviewed-by: Jocelyn Falempe <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Signed-off-by: Jocelyn Falempe <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/drm_panic_qr.rs

···
 ///    will be encoded as binary segment, otherwise it will be encoded
 ///    efficiently as a numeric segment, and appended to the URL.
 /// * `data_len`: Length of the data, that needs to be encoded, must be less
-///    than data_size.
+///    than `data_size`.
 /// * `data_size`: Size of data buffer, it should be at least 4071 bytes to hold
 ///    a V40 QR code. It will then be overwritten with the QR code image.
 /// * `tmp`: A temporary buffer that the QR code encoder will use, to write the