[Backport] Fixed Incorrect class name on Orders and returns page.

[shikhamis11]

Original Pull Request

#19784
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
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.2-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @nuzil, thank you for the review.
ENGCOM-3791 has been created to process this Pull Request

[ghost]

Hi @shikhamis11, thank you for your contribution!
Please, complete Contribution Survey, it will take less than a minute.
Your feedback will help us to improve contribution process.

[magento-engcom-team]

Hi @shikhamis11. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.