Add doc example to HashMap::hasher

Author: napen123

src/libstd/collections/hash/map.rs

@@ -691,6 +691,17 @@ impl<K, V, S> HashMap<K, V, S>
     /// Returns a reference to the map's [`BuildHasher`].
     ///
     /// [`BuildHasher`]: ../../std/hash/trait.BuildHasher.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    /// use std::collections::hash_map::RandomState;
+    ///
+    /// let hasher = RandomState::new();
+    /// let map: HashMap<isize, isize> = HashMap::with_hasher(hasher);
+    /// let hasher: &RandomState = map.hasher();
+    /// ```
     #[stable(feature = "hashmap_public_hasher", since = "1.9.0")]
     pub fn hasher(&self) -> &S {
         &self.hash_builder