Fix the documentation to use the correct variable in a code example

What does this MR do?

Actually use $CI_ENVIRONMENT_SLUG in the environment:url

Why?

Below the modified code example, it currently says:

we use $CI_ENVIRONMENT_SLUG in the environment:url so that the environment can get a specific and distinct URL for each branch (...) You could also use $CI_BUILD_REF_SLUG in environment:url, e.g.: https://$CI_BUILD_REF_SLUG.review.example.com. We use $CI_ENVIRONMENT_SLUG here because it is guaranteed to be unique

This seems to imply $CI_ENVIRONMENT_SLUG should be used, even though it uses $CI_BUILD_REF_SLUG in the example. Had me confused for a minute :)