fix javascript typo

Author: MarvinBlstrli

reference/forms/types/collection.rst

@@ -147,7 +147,7 @@ you need is this JavaScript code:
         jQuery('.add-another-collection-widget').click(function (e) {
             var list = jQuery(jQuery(this).attr('data-list'));
             // Try to find the counter of the list or use the length of the list
-            var counter = list.data('widget-counter') | list.children().length;
+            var counter = list.data('widget-counter') || list.children().length;
 
             // grab the prototype template
             var newWidget = list.attr('data-prototype');