Merge branch 'doc-ci-variables-readme-fix' into 'master'

fix table in doc/ci/variables/README.md

The table was missing a column separator, causing GitLab to render the
markdown incorrectly.

[ci skip]

See merge request !2840

doc/ci/variables/README.md

@@ -16,7 +16,7 @@ The API_TOKEN will take the Secure Variable value: `SECURE`.
 ### Predefined variables (Environment Variables)
 
 | Variable                | Runner | Description |
-|-------------------------|-------------|
+|-------------------------|-----|--------|
 | **CI**                  | 0.4 | Mark that build is executed in CI environment |
 | **GITLAB_CI**           | all | Mark that build is executed in GitLab CI environment |
 | **CI_SERVER**           | all | Mark that build is executed in CI environment |