bpo-41199: Docstring convention not followed for dataclasses documentation page (GH-21413)

Automerge-Triggered-By: @ericvsmith
(cherry picked from commit 61bb24a270)

Co-authored-by: marload <rladhkstn8@gmail.com>

Doc/library/dataclasses.rst

@@ -23,7 +23,7 @@ using :pep:`526` type annotations.  For example this code::
 
   @dataclass
   class InventoryItem:
-      '''Class for keeping track of an item in inventory.'''
+      """Class for keeping track of an item in inventory."""
       name: str
       unit_price: float
       quantity_on_hand: int = 0