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

Change example function to append rather than add lists.
(cherry picked from commit da5e947)

Co-authored-by: Tom Faulkner <[email protected]>

Author: TomFaulkner

Doc/library/dataclasses.rst

@@ -533,7 +533,7 @@ Mutable default values
      class C:
          x = []
          def add(self, element):
-             self.x += element
+             self.x.append(element)
 
      o1 = C()
      o2 = C()