Make `Vec::new` public in vec-alloc.md

src/vec/vec-alloc.md

@@ -22,7 +22,7 @@ So:
 use std::mem;
 
 impl<T> Vec<T> {
-    fn new() -> Self {
+    pub fn new() -> Self {
         assert!(mem::size_of::<T>() != 0, "We're not ready to handle ZSTs");
         Vec {
             ptr: NonNull::dangling(),