Fixed doc tests for added methods

src/liballoc/collections/btree/map.rs

@@ -2386,6 +2386,7 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
     /// # Examples
     ///
     /// ```
+    /// #![feature(or_insert_with_key)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map: BTreeMap<&str, usize> = BTreeMap::new();

src/libstd/collections/hash/map.rs

@@ -1951,6 +1951,7 @@ impl<'a, K, V> Entry<'a, K, V> {
     /// # Examples
     ///
     /// ```
+    /// #![feature(or_insert_with_key)]
     /// use std::collections::HashMap;
     ///
     /// let mut map: HashMap<&str, usize> = HashMap::new();