core: Small fix in fold docs

Adaptors are things that take iterators and adapt them into other
iterators. With this definition, fold is just a usual method, because it
doesn't normally make an iterator.

Author: bluss

src/libcore/iter/iterator.rs

@@ -1341,7 +1341,7 @@ pub trait Iterator {
         (left, right)
     }
 
-    /// An iterator adaptor that applies a function, producing a single, final value.
+    /// An iterator method that applies a function, producing a single, final value.
     ///
     /// `fold()` takes two arguments: an initial value, and a closure with two
     /// arguments: an 'accumulator', and an element. The closure returns the value that