fix(samples): Fix the loop_output.py example to handle the new parallel loop type requirement

[Tomcli]

Description of your changes:
After PR #10494, all the parallel loop input type has to be List. The current loop_output.py example still output as str which causes the following errors:

Cannot iterate over a single parameter using `dsl.ParallelFor`. Expected a list of parameters as argument to `items`.

Thus, fix the loop_output.py example to output type List[str].

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/assign @connor-mccarthy

[google-oss-prow]

@Tomcli: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipelines-samples-v2	db3dd62	link	false	/test kubeflow-pipelines-samples-v2

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[connor-mccarthy]

/lgtm
/approve

Thank you, @Tomcli!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: connor-mccarthy

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