docs: Add missing variable to QUnit.moduleDone example

Follows-up 9ab52be, in which the error was introduced.

Credit to @xdvarpunen.
Ref #1734.

docs/callbacks/QUnit.moduleDone.md

@@ -35,6 +35,6 @@ Register a callback that logs the module results
 
 ```js
 QUnit.moduleDone(details => {
-  console.log(`Finished running: ${details.name} Failed/total: ${details.failed}/${total}`);
+  console.log(`Finished running: ${details.name} Failed/total: ${details.failed}/${details.total}`);
 });
 ```