Fix: Switch sample values for $CI_SERVER_REVISION and $CI_SERVER_VERSION.

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html.

## What does this MR do?

There is a sample in this document showing the possible values of the predefined CI variables. Those of **CI_SERVER_REVISION** and **CI_SERVER_VERSION** were mixed up. I fixed that.

See merge request !6243