gst-docs: add return value to appsink callback in tutorial

The "crude waveform generator" sample application in "Basic
tutorial 8: Short-cutting the pipeline" was lacking the return value
so it was only being invoked once.

https://bugzilla.gnome.org/show_bug.cgi?id=796476

markdown/tutorials/basic/short-cutting-the-pipeline.md

@@ -499,7 +499,7 @@ Once we have the buffer ready, we pass it to `appsrc` with the
 
 ``` c
 /* The appsink has received a buffer */
-static void new_sample (GstElement *sink, CustomData *data) {
+static GstFlowReturn new_sample (GstElement *sink, CustomData *data) {
   GstSample *sample;
   /* Retrieve the buffer */
   g_signal_emit_by_name (sink, "pull-sample", &sample);
@@ -507,7 +507,9 @@ static void new_sample (GstElement *sink, CustomData *data) {
     /* The only thing we do in this example is print a * to indicate a received buffer */
     g_print ("*");
     gst_sample_unref (sample);
+    return GST_FLOW_OK;
   }
+  return GST_FLOW_ERROR;
 }
 ```