Fix doctest

Author: balt-dev

library/alloc/src/vec/mod.rs

@@ -2572,11 +2572,15 @@ impl<T, A: Allocator> Vec<T, A> {
     /// ```
     /// #![feature(push_mut)]
     ///
+    ///
     /// let mut vec = vec![1, 2];
     /// let last = vec.push_mut(3);
+    /// assert_eq!(*last, 3);
     /// assert_eq!(vec, [1, 2, 3]);
-    /// *last = 4;
-    /// assert_eq!(vec, [1, 2, 4]);
+    ///
+    /// let last = vec.push_mut(3);
+    /// *last += 1;
+    /// assert_eq!(vec, [1, 2, 3, 4]);
     /// ```
     ///
     /// # Time complexity