Fix lighthouse highlight test

[rmccar]

What is the context of this PR?

Fixes: ONSdigital/design-team#217

This updates the highlighting example so that the headings are sequential by changing the second h1 to a h2

How to review this PR

• Check the lighthouse tests in the github actions and see that the result for the test for the example-highlighting example is now 100
• Also run the test locally on this branch in chrome and on the main branch to see that this issue is now fixed.

Checklist

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• I have linked the correct Issue

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