Remove erroneous comma from example setting

Hi Joakim,

This is the same Stefan Sobering that has been emailing. I solved my issue by adding the following to my application: 

```
set :assets_precompile, %w(application.js application.css)
```

I swear I had already tried that many times, as my filenames differ from the default 'app.js' and 'app.css', but I got it working nonetheless.

However, in my quest to get it working I read the README multiple times and noticed a very small, trivial error. There was an erroneous comma in one of the setting examples, which I removed.

Cheers