bpo-42489: Fix the signature for list.sort() in the tutorial (GH-23538)

Author: ZackerySpytz

Doc/tutorial/datastructures.rst

@@ -78,7 +78,7 @@ objects:
  Return the number of times *x* appears in the list.
 
 
-.. method:: list.sort(key=None, reverse=False)
+.. method:: list.sort(*, key=None, reverse=False)
  :noindex:
 
  Sort the items of the list in place (the arguments can be used for sort