Incorrect link to reST markup example

[ThijsDrost]

In the file format.rst there is a link to http://docutils.sourceforge.net/docs/user/rst/demo.txt:

A more extensive example of reST markup can be found in `this example
document <http://docutils.sourceforge.net/docs/user/rst/demo.txt>`_;

however file extension seems to have changed to :
http://docutils.sourceforge.net/docs/user/rst/demo.rst