Fix docstring of FileLink to match parameter name

It used to be `directory_prefix`, but is currently `url_prefix`.

IPython/lib/display.py

@@ -323,7 +323,7 b' class FileLink(object):'
         ----------
         path : str
             path to the file or directory that should be formatted
-        directory_prefix : str
+        url_prefix : str
             prefix to be prepended to all files to form a working link [default:
             'files']
         result_html_prefix : str