Show the capacity in HashSet::with_capacity doc example.

src/libstd/collections/hash/set.rs

@@ -147,6 +147,7 @@ pub fn new() -> HashSet<T, RandomState> {
     /// ```
     /// use std::collections::HashSet;
     /// let set: HashSet<i32> = HashSet::with_capacity(10);
+    /// assert!(set.capacity() >= 10);
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]