Update the syntax of DAG example
What does this MR do?
The example given at https://docs.gitlab.com/ee/ci/yaml/README.html#needs does not pass CI validation with the error:
This GitLab CI configuration is invalid: (<unknown>): found unexpected ':' while scanning a plain scalar at line 9 column 11 Learn more
Syntax will work this way with double quotes:
needs: ["linux:build"]
This MR updates the example shown in the docs.