Dataclasses: Fix example on 30.6.8, add method should receive a list rather than…

Dataclasses: Fix example on 30.6.8, add method should receive a list rather than an integer. (GH-8038)

Change example function to append rather than add lists.

Doc/library/dataclasses.rst

@@ -532,7 +532,7 @@ Mutable default values
      class C:
          x = []
          def add(self, element):
-             self.x += element
+             self.x.append(element)
 
      o1 = C()
      o2 = C()