Fixed #15147 - Typo in "Managing static files"; thanks ori for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15284 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/contrib/staticfiles.txt

@@ -50,7 +50,7 @@ your additional files directory(ies) e.g.::
         "/opt/webfiles/common",
     )
 
-In case you want to refer to files in one of the locations with a additional
+In case you want to refer to files in one of the locations with an additional
 namespace, you can **optionally** provide a prefix as ``(prefix, path)``
 tuples, e.g.::