Fix link to 'what is a service' anchor

Fix invalid links to GitLab CI Docker documentation in all gitlab-ci.yml templates.

Crystal.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: "crystallang/crystal:latest"
 # Pick zero or more services to be used on all builds.
 # Only needed when using a docker container to run your tests in.
-# Check out: http://docs.gitlab.com/ce/ci/docker/using_docker_images.html#what-is-service
+# Check out: http://docs.gitlab.com/ce/ci/docker/using_docker_images.html#what-is-a-service
 # services:
 #   - mysql:latest
 #   - redis:latest