Old preview syntax in testing pipeline

[yousefmoazzam]

The old syntax for the preview parameter's value is used in the following testing pipeline:

httomo/tests/samples/pipeline_template_examples/testing/incorrect_method.yaml

Lines 8 to 12 in f46c6d6

	preview:
	-
	- start: 30
	stop: 60
	-

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 which could be confusing to any users who notice the different syntax.