pythongh-91687: modernize dataclass example typing (pythonGH-103773)

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 <35788148+alago1@users.noreply.github.com>

Doc/library/dataclasses.rst

@@ -710,7 +710,7 @@ 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