Fix documented signature of is_mapping()

Author: mwilliamson

README.rst

@@ -106,7 +106,7 @@ For instance, ``has_attrs(name="bob")`` is equivalent to ``has_attrs(name=equal_
       # Matches [42], [42, 42, 42] and []
       # but not [42, 43]
 
-* ``is_mapping(**matchers)``: matches a mapping, such as a ``dict``, if it has the same keys with matching values.
+* ``is_mapping(matchers)``: matches a mapping, such as a ``dict``, if it has the same keys with matching values.
   An error will be raised if the mapping is missing any keys, or has any extra keys.
   For instance: