Correctly describe this as a Set not a Map in docs (#100)

diff --git a/lib/src/equality_set.dart b/lib/src/equality_set.dart
index 6f6a426..f4abec6 100644
--- a/lib/src/equality_set.dart
+++ b/lib/src/equality_set.dart
@@ -7,7 +7,7 @@
 import 'equality.dart';
 import 'wrappers.dart';
 
-/// A [Map] whose key equality is determined by an [Equality] object.
+/// A [Set] whose key equality is determined by an [Equality] object.
 class EqualitySet<E> extends DelegatingSet<E> {
   /// Creates a set with equality based on [equality].
   EqualitySet(Equality<E> equality)