fix broken doc link
The docs tried to refer to a top-level 'defer' function; instead link to
Guard::defer.
//! [`Collector`]: struct.Collector.html
//! [`Shared`]: struct.Shared.html
//! [`pin`]: fn.pin.html
-//! [`defer`]: fn.defer.html
+//! [`defer`]: struct.Guard.html#method.defer
#![warn(missing_docs)]
#![warn(missing_debug_implementations)]
