rust: rbtree: fix overindented list item

Starting with Rust 1.86.0 (to be released 2025-04-03), Clippy will have
a new lint, `doc_overindented_list_items` [1], which catches cases of
overindented list items.

The lint has been added by Yutaro Ohno, based on feedback from the kernel
[2] on a patch that fixed a similar case -- commit 0c5928de ("rust:
block: fix formatting in GenDisk doc").

Clippy reports a few cases in the kernel, apart from the one already
fixed in the commit above. One is this one:

    error: doc list item overindented
        --> rust/kernel/rbtree.rs:1152:5
         |
    1152 | ///     null, it is a pointer to the root of the [`RBTree`].
         |     ^^^^ help: try using `  ` (2 spaces)
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items
         = note: `-D clippy::doc-overindented-list-items` implied by `-D warnings`
         = help: to override `-D warnings` add `#[allow(clippy::doc_overindented_list_items)]`

Thus clean it up.

Cc: Yutaro Ohno <yutaro.ono.418@gmail.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Fixes: a335e959 ("rust: rbtree: add `RBTree::entry`")
Link: https://github.com/rust-lang/rust-clippy/pull/13711 [1]
Link: https://github.com/rust-lang/rust-clippy/issues/13601

 [2]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Yutaro Ohno <yutaro.ono.418@gmail.com>
Link: https://lore.kernel.org/r/20250206232022.599998-1-ojeda@kernel.org


[ There are a few other cases, so updated message. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>