add trial duration to example code

docs/overview/dynamic-parameters.md

@@ -32,6 +32,7 @@ var trial = {
 
 var feedback = {
   type: jsPsychHtmlKeyboardResponse,
+  trial_duration: 1000,
   stimulus: function(){
     // The feedback stimulus is a dynamic parameter because we can't know in advance whether
     // the stimulus should be 'correct' or 'incorrect'.