Fix typo

Fix typo extra ` . 
`newstr`` --> `newstr`

Author: nikochiko

docs/str_object.rst

@@ -14,7 +14,7 @@ There are also other differences, such as the ``repr`` of unicode strings in
 Py2 having a ``u'...'`` prefix, versus simply ``'...'``, and the removal of
 the :func:`str.decode` method in Py3.
 
-:mod:`future` contains a :class:`newstr`` type that is a backport of the
+:mod:`future` contains a :class:`newstr` type that is a backport of the
 :mod:`str` object from Python 3. This inherits from the Python 2
 :class:`unicode` class but has customizations to improve compatibility with
 Python 3's :class:`str` object. You can use it as follows::