Sync-up parameter name in equivalent code snippet of `enumerate` (GH-101029)

Doc/library/functions.rst

@@ -505,9 +505,9 @@ are always available.  They are listed here in alphabetical order.
 
    Equivalent to::
 
-      def enumerate(sequence, start=0):
+      def enumerate(iterable, start=0):
           n = start
-          for elem in sequence:
+          for elem in iterable:
               yield n, elem
               n += 1