Added code example for setenv class

Example usage is based on the Apache docs' own example for the SetEnv directive: 
http://httpd.apache.org/docs/2.2/mod/mod_env.html#setenv

Change requested here:
https://tickets.puppetlabs.com/browse/DOCUMENT-204
- The code blocks in readmes are typically indented 4 spaces, but this is 5. It doesn't really change anything and could even be indented 0 spaces, but markdown typically expects 4 spaces to turn on block-level preformatted text (though github markdown uses the ``` to do so).
- There should be one space after the `{` before the title.
- Needs two space indentation for the attribute line.
- The `}` should be indented to line up with the a in `apache::vhost` above.
- Because there are no other parameters, the `=>` should have a single space after `setenv` (arrow alignment as per https://docs.puppetlabs.com/guides/style_guide.html#arrow-alignment).

The apache readme has a bunch of mis-formatted examples already; I wish we had a way to lint examples and not just code.