Fix order in jvm options in `java -jar` sample

Hello,

I just fixed the order in the sample [as suggested](https://github.com/weavejester/environ/issues/12).

Just out of curiosity, is there a reason not to factorize the [common behavior between the private functions `read-system-env` and `read-system-props`](https://github.com/weavejester/environ/blob/master/environ/src/environ/core.clj#L18-L24)?

Cheers,