Incorrect online doc reference to infinispan.xml dtd

Hi guys,

Just discovered that the referenced config DTD docs are for the old 5.1 version.

See http://infinispan.org/docs/6.0.x/user_guide/user_guide.html#_configuring_cache_declaratively

"""Refer to Infinispan configuration reference for more details. If you are using XML editing tools for configuration writing you can use provided Infinispan schema to assist you."""