Use singular they in json docs

Same as rust-lang/rust#21314 but for rustc-serialize instead of serialize.

Author: fenhl

src/json.rs

@@ -40,7 +40,7 @@
 //! 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
+//! their age, address and phone numbers could look like
 //!
 //! ```ignore
 //! {