Tiny tweak to Iterator::unzip() doc comment example.

It's easier to figure out what it's doing and which output
elements map to which input ones if the matrix we are dealing
with is rectangular 2x3 rather than square 2x2.

library/core/src/iter/traits/iterator.rs

@@ -2837,12 +2837,12 @@ pub trait Iterator {
     /// Basic usage:
     ///
     /// ```
-    /// let a = [(1, 2), (3, 4)];
+    /// let a = [(1, 2), (3, 4), (5, 6)];
     ///
     /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
     ///
-    /// assert_eq!(left, [1, 3]);
-    /// assert_eq!(right, [2, 4]);
+    /// assert_eq!(left, [1, 3, 5]);
+    /// assert_eq!(right, [2, 4, 6]);
     ///
     /// // you can also unzip multiple nested tuples at once
     /// let a = [(1, (2, 3)), (4, (5, 6))];