Merge branch 'docs/update-variables-image' into 'master'

Replace old "secret variables" image for variables with new "variables" image

See merge request gitlab-org/gitlab-ce!22184

doc/ci/variables/README.md

@@ -197,7 +197,7 @@ Likewise, group-level variables can be added by going to your group's
 **Settings > CI/CD**, then finding the section called **Variables**.
 Any variables of [subgroups] will be inherited recursively.
-![Variables](img/secret_variables.png)
+![Variables](img/variables.png)
 Once you set them, they will be available for all subsequent pipelines. You can also
 [protect your variables](#protected-variables).