Update vec.rs path

vec.rs has moved https://github.com/rust-lang/rust/blob/master/src/liballoc/vec.rs

Relevant commit from 2017 eadda7665e (diff-541a0d1eb7d496a1f91ed729f52261ed)

idioms/ctor.md

@@ -9,7 +9,7 @@ is to use a static `new` method to create an object.
 ## Example
 
 ```rust
-// A Rust vector, see libcollections/vec.rs
+// A Rust vector, see liballoc/vec.rs
 pub struct Vec<T> {
     buf: RawVec<T>,
     len: usize,