Update examples with AWS CLI and Ruby CLI

* add a `sh` mark to have a nice highlighting (see [Syntax highlighting](https://help.github.com/articles/github-flavored-markdown/#syntax-highlighting)).

* move `Using AWS CLI:` and `EMR Ruby CLI:` out of code blocks.

* split commands on a few lines to avoid scroll bars.

* wrap `MYKEY` into quotation marks `<MYKEY>`, to explicitly note that user should replace it.