Arrow functions are not instanceof Function

But their `typeof` is `function`, so use this instead to
detect whether an argument is a function which can be
called.

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/qyx7en/

src/web_accessible_resources/amazon_apstag.js

@@ -21,6 +21,7 @@
 
 // https://www.reddit.com/r/uBlockOrigin/comments/ghjqph/
 // https://github.com/NanoMeow/QuickReports/issues/3717
+// https://www.reddit.com/r/uBlockOrigin/comments/qyx7en/
 
 (function() {
     'use strict';
@@ -30,7 +31,7 @@
     }.bind();
     const apstag = {
         fetchBids: function(a, b) {
-            if ( b instanceof Function ) {
+            if ( typeof b === 'function' ) {
                 b([]);
             }
         },