Fix wrong naming of services keyword [ci skip]

doc/ci/services/README.md

 ## GitLab CI Services
 
-GitLab CI uses the `service` keyword to define what docker containers should be
+GitLab CI uses the `services` keyword to define what docker containers should be
 linked with your base image. Below is a list of examples you may use.
 
 + [Using MySQL](mysql.md)