drm/panic: fix overindented list items in documentation

Starting with the upcoming Rust 1.86.0 (to be released 2025-04-03),
Clippy warns:

    error: doc list item overindented
       --> drivers/gpu/drm/drm_panic_qr.rs:914:5
        |
    914 | ///    will be encoded as binary segment, otherwise it will be encoded
        |     ^^^ help: try using `  ` (2 spaces)
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items



The overindentation is slightly hard to notice, since all the items
start with a backquote that makes it look OK, but it is there.

Thus fix it.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Fixes: cb5164ac ("drm/panic: Add a QR code panic screen")
Cc: stable@vger.kernel.org # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250301231602.917580-2-ojeda@kernel.org