pythongh-91687: modernize dataclass example typing (python#103773)

modernize dataclass example typing `list` rather than `List` and comment as to that line being the alluded too error.

Doc/library/dataclasses.rst

@@ -714,7 +714,7 @@ 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