doc: "mut" not needed for the examples

Author: tshepang

src/doc/book/vectors.md

@@ -116,11 +116,11 @@ for i in v {
 ```
 
 Note: You cannot use the vector again once you have iterated by taking ownership of the vector.
-You can iterate the vector multiple times by taking a reference to the vector whilst iterating. 
+You can iterate the vector multiple times by taking a reference to the vector whilst iterating.
 For example, the following code does not compile.
 
 ```rust,ignore
-let mut v = vec![1, 2, 3, 4, 5];
+let v = vec![1, 2, 3, 4, 5];
 
 for i in v {
     println!("Take ownership of the vector and its element {}", i);
@@ -134,7 +134,7 @@ for i in v {
 Whereas the following works perfectly,
 
 ```rust
-let mut v = vec![1, 2, 3, 4, 5];
+let v = vec![1, 2, 3, 4, 5];
 
 for i in &v {
     println!("This is a reference to {}", i);