rust: macros: fix documentation of the paste! macro

One of the example in this section uses a curious mix of the constant
and function declaration syntaxes; fix it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 823d473 ("rust: macros: add `paste!` proc macro")
Link: https://lore.kernel.org/r/20241019072208.1016707-1-pbonzini@redhat.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: bonzini

rust/macros/lib.rs

@@ -359,7 +359,7 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 /// macro_rules! pub_no_prefix {
 ///     ($prefix:ident, $($newname:ident),+) => {
 ///         kernel::macros::paste! {
-///             $(pub(crate) const fn [<$newname:lower:span>]: u32 = [<$prefix $newname:span>];)+
+///             $(pub(crate) const fn [<$newname:lower:span>]() -> u32 { [<$prefix $newname:span>] })+
 ///         }
 ///     };
 /// }