bpo-29952: Use usual terminology of dict (GH-917)

s/keys and elements/keys and values/

Doc/reference/expressions.rst

@@ -1355,7 +1355,7 @@ built-in types.
     true).
 
 * Mappings (instances of :class:`dict`) compare equal if and only if they have
-  equal `(key, value)` pairs. Equality comparison of the keys and elements
+  equal `(key, value)` pairs. Equality comparison of the keys and values
   enforces reflexivity.
 
   Order comparisons (``<``, ``>``, ``<=``, and ``>=``) raise :exc:`TypeError`.