Fix typo: refered -> referred in doc comment (#11)

Author: Jah-yee

src/identifiable.rs

@@ -1,6 +1,6 @@
 use super::IdClass;
 
-/// An identifiable is an object which can be refered to with
+/// An identifiable is an object which can be referred to with
 /// an id (the id may be part, or not, of the object).
 ///
 /// The best way to add this trait to a struct is to