In the steps, the returns tab has {{ pages.returns.content }}, which basically gets the contents of the page with the handle ‘returns’
If you go to your returns policy page in your Shopify admin, you can view or edit its handler by clicking on ‘Edit website SEO’ near the bottom, under ‘URL and handle’, and you can change the handle name at the end of the textfield to ‘returns’.
Alternatively you can also change the code itself e.g. to {{ pages.returns-policy.content }} instead of changing the handle.
For more options and without needing to go into code, you can try out our app to add tabs / accordion to your product page - https://apps.shopify.com/custom-product-accordion-tabs =)