Fixed copy/paste error in measurement docs

.. class:: Area(**kwargs)

   To initialize a distance object, pass in a keyword corresponding to

   To initialize an area object, pass in a keyword corresponding to

   the desired :ref:`unit attribute name <supported_units>` set with

   desired value.  For example, the following creates a distance

   desired value.  For example, the following creates an area

   object representing 5 square miles::

       >>> a = Area(sq_mi=5)