Minor Allocator doc fix

src/liballoc/allocator.rs

@@ -370,7 +370,6 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     }
 }
 
-/// An implementation of `Allocator` can allocate, reallocate, and
 /// An implementation of `Alloc` can allocate, reallocate, and
 /// deallocate arbitrary blocks of data described via `Layout`.
 ///