Updated xhtml2pdf name/URL in docs.

* PDFlib_ is another PDF-generation library that has Python bindings. To

  use it with Django, just use the same concepts explained in this article.

* `Pisa XHTML2PDF`_ is yet another PDF-generation library. Pisa ships with

  an example of how to integrate Pisa with Django.

* `XHTML2PDF`_ is yet another PDF-generation library. It ships with an example

  of how to integrate it with Django.

* HTMLdoc_ is a command-line script that can convert HTML to PDF. It

  doesn't have a Python interface, but you can escape out to the shell

  using ``system`` or ``popen`` and retrieve the output in Python.

.. _PDFlib: http://www.pdflib.org/

.. _`Pisa XHTML2PDF`: http://www.xhtml2pdf.com/

.. _XHTML2PDF: https://github.com/xhtml2pdf/xhtml2pdf

.. _HTMLdoc: https://www.msweet.org/projects.php?Z1

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