Fix a link (#347)

doc/index.rst

@@ -24,8 +24,8 @@ Format API
 The replacement-based Format API provides a safe alternative to ``printf``,
 ``sprintf`` and friends with comparable or `better performance
 <http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_.
-The `format string syntax <doc/latest/index.html#format-string-syntax>`_ is similar
-to the one used by `str.format <http://docs.python.org/2/library/stdtypes.html#str.format>`_
+The `format string syntax <syntax.html>`_ is similar to the one used by
+`str.format <http://docs.python.org/2/library/stdtypes.html#str.format>`_
 in Python:
 
 .. code:: c++