Document to use name: review/$CI_COMMIT_REF_SLUG instead of review/$CI_COMMIT_REF_NAME for review apps

Problem

Pointed out by the comment by @hfyngvason.

Currently, our documentation recommends to use environment: review/$CI_COMMIT_REF_NAME for configuring review apps, however, this sometimes causes an environment creation error by an invalid name. Users should use $CI_COMMIT_REF_SLUG instead, that automatically sanitizes/normalizes the invalid invalid characters.

Proposal

Replace environment: review/$CI_COMMIT_REF_NAME by environment: review/$CI_COMMIT_REF_SLUG.