Use an existing image for testing /help

What does this MR do?

In spec/controllers/help_controller_spec.rb we test how an image is rendered under /help. Given that we usually rename images as we update them, we should use one that remains unchanged.

!24139 (merged) fixed a failure and introduced a new image. Let's use one that we already use in the docs and we're unlikely to change its name.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done