Merge branch 'doc-ci-anchor-yaml-fix' into 'master'

Fix CI yaml example

## What does this MR do?

Fixes a  typo in a CI yaml example.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19888

See merge request !5304

doc/ci/yaml/README.md

@@ -985,11 +985,11 @@ directive defined in `.postgres_services` and `.mysql_services` respectively:
     - ruby
 
 test:postgres:
-  << *job_definition
+  <<: *job_definition
   services: *postgres_definition
 
 test:mysql:
-  << *job_definition
+  <<: *job_definition
   services: *mysql_definition
 ```