Merge branch 'patch-5' into 'master'

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

doc/ci/variables/README.md

@@ -76,8 +76,8 @@ export CI_RUNNER_DESCRIPTION="my runner"
 export CI_RUNNER_TAGS="docker, linux"
 export CI_SERVER="yes"
 export CI_SERVER_NAME="GitLab"
-export CI_SERVER_REVISION="8.9.0"
-export CI_SERVER_VERSION="70606bf"
+export CI_SERVER_REVISION="70606bf"
+export CI_SERVER_VERSION="8.9.0"
 ```
 
 ### YAML-defined variables