fixed browser-check typo in doc demos

Author: Zhaobin Li

docs/demos/jspsych-browser-check-demo2.html

@@ -17,7 +17,9 @@
       inclusion_function: (data) => {
         return ['chrome', 'firefox'].includes(data.browser);
       },
-      exclusion_message: `<p>You must use Chrome or Firefox to complete this experiment.</p>`
+      exclusion_message: (data) => {
+            return `<p>You must use Chrome or Firefox to complete this experiment.</p>`
+      },
     };
 
     const timeline = [trial];