docs: pwg: fix typo in 'Dynamic negotiation' section

The point of this example is to show how to set caps
on the source pad once it has been set on the sink pad.
So, in passthrough mode, the caps is just copied to the
source pad.

https://bugzilla.gnome.org/show_bug.cgi?id=738153

docs/pwg/advanced-negotiation.xml

@@ -370,7 +370,7 @@ static gboolean
 gst_my_filter_setcaps (GstMyFilter *filter,
 		       GstCaps *caps)
 {
-  if (gst_pad_set_caps (filter->sinkpad, caps)) {
+  if (gst_pad_set_caps (filter->srcpad, caps)) {
     filter->passthrough = TRUE;
   } else {
     GstCaps *othercaps, *newcaps;