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

A new style lint 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.

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

rust/macros/lib.rs

@@ -312,7 +312,7 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 ///
 /// Currently supported modifiers are:
 /// * `span`: change the span of concatenated identifier to the span of the specified token. By
-/// default the span of the `[< >]` group is used.
+///   default the span of the `[< >]` group is used.
 /// * `lower`: change the identifier to lower case.
 /// * `upper`: change the identifier to upper case.
 ///