qtmux: Fix fourcc for ProRes Proxy

This is apco, according to
https://wiki.multimedia.cx/index.php?title=Apple_ProRes

https://bugzilla.gnome.org/show_bug.cgi?id=769048

gst/isomp4/gstqtmux.c

@@ -4214,7 +4214,7 @@ gst_qt_mux_video_sink_set_caps (GstQTPad * qtpad, GstCaps * caps)
     else if (!g_strcmp0 (variant, "hq"))
       entry.fourcc = FOURCC_apch;
     else if (!g_strcmp0 (variant, "proxy"))
-      entry.fourcc = FOURCC_ap4h;
+      entry.fourcc = FOURCC_apco;
   }
 
   if (!entry.fourcc)