Fix link to 'what is a service' anchor

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

Django.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: python: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
   - postgres:latest

Elixir.gitlab-ci.yml

@@ -2,7 +2,7 @@ image: elixir: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

Laravel.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: php: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

Nodejs.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: node: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

Ruby.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: "ruby:2.3"
 # 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

Rust.gitlab-ci.yml

@@ -4,7 +4,7 @@ image: "scorpil/rust:stable"
 # Optional: 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