Update TimeMachineGrowler.m

Of course, I make this change months ago, and then the day after I tell you about it on Twitter, it starts acting funny :/.  Basically the problem now is that after the first notification, sometimes it crashes with a EXC_BAD_ACCESS exception.  Other times, it will function just fine.

Halfway there I guess...

Sorry if this is a worthless pull.

![screen shot 2013-08-25 at 6 14 16 pm](https://f.cloud.github.com/assets/5308569/1023644/ab788f7c-0dfb-11e3-81e0-0aee9140a1f1.png)