typo in the assets documentation: local_assets_dir

According to the `asset.rb` where you have:

```
system("rsync -a --del -L -K -vv --progress --rsh='ssh -p #{port}' ./#{dir} #{user}@#{server}:#{cap.current_path}/#{cap.fetch(:local_assets_dir)}")
```

The variable `local_assets_dir`expects a string, not an array like in the docs:

```
set :local_assets_dir, %w(public/assets public/att)
```

Please fix it. 
