rust: macros: indent list item in `paste!`'s docs

A new style lint, `doc_lazy_continuation` [1], has been introduced in the
upcoming Rust 1.80 (currently in beta), which detects missing indentation
in code documentation.

We have one such case:

error: doc list item missing indentation
--> rust/macros/lib.rs:315:5
|
315 | /// default the span of the `[< >]` group is used.
| ^
|
= help: if this is supposed to be its own paragraph, add a blank line
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_lazy_continuation
= note: `-D clippy::doc-lazy-continuation` implied by `-D clippy::style`
= help: to override `-D clippy::style` add `#[allow(clippy::doc_lazy_continuation)]`
help: indent this line
|
315 | /// default the span of the `[< >]` group is used.
| ++

While the rendering of the docs by `rustdoc` is not affected, we apply
this kind of indentation elsewhere since it looks better.

Thus clean it up.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#/doc_lazy_continuation [1]
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Finn Behrens <me@kloenk.dev>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20240709160615.998336-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>