Fix invalid example for database dictionary yml file

What does this MR do?

The example for a database dictionary file in the official documentation is errorneous, see https://docs.gitlab.com/ee/development/database/database_dictionary.html#example-dictionary-file . When using this example and running rails db:migrate it will lead to the following unexpected and irritating error. After a few moments, you realize that yaml file should have only three dash characters (---) and not four ---- ;-). This MR fixes the example to avoid confusion in the future.

StandardError: An error has occurred, this and all later migrations canceled:

(<unknown>): mapping values are not allowed in this context at line 2 column 11
/Users/client-siemens/Development/gitlab-development-kit/gitlab/lib/gitlab/database/gitlab_schema.rb:129:in `block in build_dictionary'
/Users/client-siemens/Development/gitlab-development-kit/gitlab/lib/gitlab/database/gitlab_schema.rb:128:in `each'
/Users/client-siemens/Development/gitlab-development-kit/gitlab/lib/gitlab/database/gitlab_schema.rb:128:in `each_with_object'
...

Related issues

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