Added instructions for use of nm on OS X

Hi Steve,

```nm``` on OS X Lion doesn't have a ```-C``` flag, and as far as I can tell it is de-mangling symbols by default. I've updated the bit in chapter 5 to reflect this.

I'm not super au fait with tools like this however (first foray into anything remotely 'low-level'), so I could be horribly wrong! :sweat: