doc: reduce indentation of examples to 4 spaces

Also, add trailing commas

src/liballoc/rc.rs

@@ -102,13 +102,13 @@
 //!
 //! struct Owner {
 //!     name: String,
-//!     gadgets: RefCell<Vec<Weak<Gadget>>>
+//!     gadgets: RefCell<Vec<Weak<Gadget>>>,
 //!     // ...other fields
 //! }
 //!
 //! struct Gadget {
 //!     id: i32,
-//!     owner: Rc<Owner>
+//!     owner: Rc<Owner>,
 //!     // ...other fields
 //! }
 //!
@@ -119,7 +119,7 @@
 //!     let gadget_owner : Rc<Owner> = Rc::new(
 //!         Owner {
 //!             name: "Gadget Man".to_string(),
-//!                 gadgets: RefCell::new(Vec::new())
+//!             gadgets: RefCell::new(Vec::new()),
 //!         }
 //!     );
 //!