rust: block: fix formatting in GenDisk doc

Align bullet points and improve indentation in the `Invariants` section
of the `GenDisk` struct documentation for better readability.

[ Yutaro is also working on implementing the lint we suggested to catch
  this sort of issue in upstream Rust:

    https://github.com/rust-lang/rust-clippy/issues/13601
    https://github.com/rust-lang/rust-clippy/pull/13711



  Thanks a lot! - Miguel ]

Fixes: 3253aba3 ("rust: block: introduce `kernel::block::mq` module")
Signed-off-by: Yutaro Ohno <yutaro.ono.418@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/ZxkcU5yTFCagg_lX@ohnotp


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>