Update usage.rst
There is a spelling error on the file "doc/source/usage.rst" at line 46.
Please change "paramter" to "parameter".
@@ -43,7 +43,7 @@ Using xhtml2pdf in Django
----------------------------
To allow URL references to be resolved using Django's STATIC_URL and MEDIA_URL settings,
-xhtml2pdf allows users to specify a ``link_callback`` paramter to point to a function
+xhtml2pdf allows users to specify a ``link_callback`` parameter to point to a function
that converts relative URLs to absolute system paths.
.. code:: python
