fix typo

debian/changelog

@@ -42,7 +42,7 @@ apt (0.9.12) unstable; urgency=low
   * Set the default "Acquire::PDiffs::FileLimit" to 20. If the amount
     of pdiffs is bigger things tend to get slower. Set
       Acquire::PDiffs::FileLimit "0";
-    in /etc/apt/preferences to get the old behavior back.
+    in /etc/apt/apt.conf to get the old behavior back.
 
  -- Michael Vogt <mvo@debian.org>  Wed, 09 Oct 2013 20:50:03 +0200