Utilize if..let for get_mut doc-comment examples

Author: frewsxcv

src/libcollections/btree/map.rs

@@ -260,9 +260,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
     ///
     /// let mut map = BTreeMap::new();
     /// map.insert(1, "a");
-    /// match map.get_mut(&1) {
-    ///     Some(x) => *x = "b",
-    ///     None => (),
+    /// if let Some(x) = map.get_mut(&1) {
+    ///     *x = "b";
     /// }
     /// assert_eq!(map[&1], "b");
     /// ```

src/libcollections/vec_deque.rs

@@ -223,11 +223,8 @@ impl<T> VecDeque<T> {
     /// buf.push_back(3);
     /// buf.push_back(4);
     /// buf.push_back(5);
-    /// match buf.get_mut(1) {
-    ///     None => {}
-    ///     Some(elem) => {
-    ///         *elem = 7;
-    ///     }
+    /// if let Some(elem) = buf.get_mut(1) {
+    ///     *elem = 7;
     /// }
     ///
     /// assert_eq!(buf[1], 7);

src/libcollections/vec_map.rs

@@ -47,9 +47,8 @@ use vec::Vec;
 ///
 /// assert_eq!(months.get(&1), Some(&"Jan"));
 ///
-/// match months.get_mut(&3) {
-///     Some(value) => *value = "Venus",
-///     None => (),
+/// if let Some(value) = months.get_mut(&3) {
+///     *value = "Venus";
 /// }
 ///
 /// assert_eq!(months.get(&3), Some(&"Venus"));
@@ -528,9 +527,8 @@ impl<V> VecMap<V> {
     ///
     /// let mut map = VecMap::new();
     /// map.insert(1, "a");
-    /// match map.get_mut(&1) {
-    ///     Some(x) => *x = "b",
-    ///     None => (),
+    /// if let Some(x) = map.get_mut(&1) {
+    ///     *x = "b";
     /// }
     /// assert_eq!(map[1], "b");
     /// ```

src/libstd/collections/hash/map.rs

@@ -1067,9 +1067,8 @@ impl<K, V, S> HashMap<K, V, S>
     ///
     /// let mut map = HashMap::new();
     /// map.insert(1, "a");
-    /// match map.get_mut(&1) {
-    ///     Some(x) => *x = "b",
-    ///     None => (),
+    /// if let Some(x) = map.get_mut(&1) {
+    ///     *x = "b";
     /// }
     /// assert_eq!(map[&1], "b");
     /// ```