docs: Fix typo

src/libcore/vec.rs

@@ -928,7 +928,7 @@ pub fn retain<T>(v: &mut ~[T], f: pure fn(t: &T) -> bool) {
  * # Arguments
  * * `z` - initial accumulator value
  * * `v` - vector to iterate over
- * * `p` - a closure to do operate on vector elements
+ * * `p` - a closure to operate on vector elements
  *
  * # Examples
  *