docs: make HashSet::retain doctest more clear

Author: Ezrashaw

library/std/src/collections/hash/set.rs

@@ -317,7 +317,7 @@ impl<T, S> HashSet<T, S> {
     ///
     /// let mut set = HashSet::from([1, 2, 3, 4, 5, 6]);
     /// set.retain(|&k| k % 2 == 0);
-    /// assert_eq!(set.len(), 3);
+    /// assert_eq!(set, HashSet::from([2, 4, 6]));
     /// ```
     ///
     /// # Performance