Fix a small typo in docs/upload_handling.txt

Most Web sites wouldn't be complete without a way to upload files. When Django

handles a file upload, the file data ends up placed in ``request.FILES`` (for

more on the ``request`` object see the documentation for `request and response

objects`_). This document explains how files are stored on disk an in memory,

objects`_). This document explains how files are stored on disk and in memory,

and how to customize the default behavior.

.. _request and response objects: ../request_response/#attributes