Added missing close-curly-brace to default value of execute_command in Chef Solo docs

This is a minor typo, but may cause brief trouble if a user copy-pastes and edits the default value of execute_command from  without looking too hard. (Guilty.) For example:

```
$ packer -debug build ami_chef.json
amazon-ebs output will be in this color.

1 error(s) occurred:

* Error parsing execute_command: template: template:1: unexpected "}" in operand; missing space?
```