Fixed Incorrect class name on Orders and returns page.

[shikhamis11]

Fixed #19780
Incorrect class name on Orders and returns page.

Summary (*)

On 'Orders and returns' page legend tag has class admin__legend as on admin area.
Examples (*)

app/code/Magento/Sales/view/frontend/templates/guest/form.phtml

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @shikhamis11. Thank you for your contribution
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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3699 has been created to process this Pull Request

[magento-engcom-team]

Hi @shikhamis11. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[magento-engcom-team]

Hi @shikhamis11. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.