[3.11] gh-91687: modernize dataclass example typing (GH-103773) (#103774

)

modernize dataclass example typing `list` rather than `List` and comment
as to that line being the alluded too error.
(cherry picked from commit 7ef614c)

Co-authored-by: Allan Lago <[email protected]>
Co-authored-by: Łukasz Langa <[email protected]>

Doc/library/dataclasses.rst

@@ -712,7 +712,7 @@ Using dataclasses, *if* this code was valid::
 
   @dataclass
   class D:
-      x: List = []
+      x: list = []      # This code raises ValueError
       def add(self, element):
           self.x += element