doc: Fix typo

src/doc/book/references-and-borrowing.md

@@ -85,7 +85,7 @@ fn main() {
     fn sum_vec(v: &Vec<i32>) -> i32 {
         return v.iter().fold(0, |a, &b| a + b);
     }
-    // Borrow two vectors and and sum them.
+    // Borrow two vectors and sum them.
     // This kind of borrowing does not allow mutation to the borrowed.
     fn foo(v1: &Vec<i32>, v2: &Vec<i32>) -> i32 {
         // do stuff with v1 and v2