(doc) Fix typo in `Handler` description
//!
//! A `Handler` will produce a `Response` given a `Request`. Most handlers are
//! functions or closures that accept a `&mut Request` as an argument and return
-//! an `IronError` containing a `Response`. A `IronError` is returned instead of
+//! an `IronResult` containing a `Response`. An `IronResult` is returned instead of
//! directly returning a `Response` in order to indicate a possibility of
//! failure (e.g. database timeout).
//!
