Old preview syntax in testing pipeline

The old syntax for the `preview` parameter's value is used in the following testing pipeline: https://github.com/DiamondLightSource/httomo/blob/f46c6d66accc78b5340684d987eb6f7451a81baa/tests/samples/pipeline_template_examples/testing/incorrect_method.yaml#L8-L12

It's not such a big deal to only be in a testing pipeline, but the main issue is that this pipeline is shown in the [YAML checker docs](https://diamondlightsource.github.io/httomo/utilities/yaml_checker.html#usage) which could be confusing to any users who notice the different syntax.

Inspect tests/samples/pipeline_template_examples/testing/incorrect_method.yaml at lines 8-12, then compare it with the YAML checker documentation's usage example. Update the outdated preview syntax so the testing pipeline and the documented example use the same current form.

Written by the indexing model from the issue text.