Update json.rs

Treemap should be BTreeMap

Author: brianloveswords

src/libserialize/json.rs

@@ -29,7 +29,7 @@
 //! * `String`: equivalent to rust's `String`
 //! * `Array`: equivalent to rust's `Vec<T>`, but also allowing objects of different types in the
 //!   same array
-//! * `Object`: equivalent to rust's `Treemap<String, json::Json>`
+//! * `Object`: equivalent to rust's `BTreeMap<String, json::Json>`
 //! * `Null`
 //!
 //! An object is a series of string keys mapping to values, in `"key": value` format.