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:

    rust-lang/rust-clippy#13601
    rust-lang/rust-clippy#13711

  Thanks a lot! - Miguel ]

Fixes: 3253aba ("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>

Author: ohno418

rust/kernel/block/mq/gen_disk.rs

@@ -174,9 +174,9 @@ impl GenDiskBuilder {
 ///
 /// # Invariants
 ///
-///  - `gendisk` must always point to an initialized and valid `struct gendisk`.
-///  - `gendisk` was added to the VFS through a call to
-///     `bindings::device_add_disk`.
+/// - `gendisk` must always point to an initialized and valid `struct gendisk`.
+/// - `gendisk` was added to the VFS through a call to
+///   `bindings::device_add_disk`.
 pub struct GenDisk<T: Operations> {
     _tagset: Arc<TagSet<T>>,
     gendisk: *mut bindings::gendisk,