Vec doc use elision as code rather than comment

Author: pickfire

library/alloc/src/vec.rs

@@ -171,7 +171,9 @@ use crate::raw_vec::RawVec;
 ///
 /// // ... and that's all!
 /// // you can also do it like this:
-/// let u: &[usize] = &v; // or &[_]
+/// let u: &[usize] = &v;
+/// // or like this:
+/// let u: &[_] = &v;
 /// ```
 ///
 /// In Rust, it's more common to pass slices as arguments rather than vectors