Use singular they in the serialize::json docs

See [https://gist.github.com/0xabad1dea/8870b192fd1758743f66](this document) by @0xabad1dea for the rationale.

Author: fenhl

src/libserialize/json.rs

@@ -34,7 +34,7 @@
 //!
 //! An object is a series of string keys mapping to values, in `"key": value` format.
 //! Arrays are enclosed in square brackets ([ ... ]) and objects in curly brackets ({ ... }).
-//! A simple JSON document encoding a person, his/her age, address and phone numbers could look like
+//! A simple JSON document encoding a person, their age, address and phone numbers could look like
 //!
 //! ```ignore
 //! {
@@ -192,8 +192,8 @@
 //! }
 //! ```
 
-use self::JsonEvent::*;
 use self::ErrorCode::*;
+use self::JsonEvent::*;
 use self::ParserError::*;
 use self::DecoderError::*;
 use self::ParserState::*;