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: 823d4737 ("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>