Use square brackets in vec! macro.

It's the default behavior of `rust-fmt`, and it's consistent with the rest
of examples from the book.

Author: tomasz-rozanski

src/std/hash/hashset.md

@@ -36,8 +36,8 @@ Try all of these in the following example:
 use std::collections::HashSet;
 
 fn main() {
-    let mut a: HashSet<i32> = vec!(1i32, 2, 3).into_iter().collect();
-    let mut b: HashSet<i32> = vec!(2i32, 3, 4).into_iter().collect();
+    let mut a: HashSet<i32> = vec![1i32, 2, 3].into_iter().collect();
+    let mut b: HashSet<i32> = vec![2i32, 3, 4].into_iter().collect();
 
     assert!(a.insert(4));
     assert!(a.contains(&4));