Remove this.sandbox.useFakeTimers() in QUnit docs

This bit me, it looks like the sinon qunit plugin has `useFakeTimers:
true` at the global level. This means that this line is redundant, and
in fact it causes tests to fail. I believe my change illustrates the
correct usage of fake timers when using the QUnit Sinon plugin.
Merge pull request #28 from mattbaker/master

Remove this.sandbox.useFakeTimers() in QUnit docs