Fix spelling in AsynchronousOperations and Backend

[hugovk]

Description (*)

Correct spelling mistakes in AsynchronousOperations and Backend.

Split out from #18442 in order to make it more manageable and less merge-conflicty.

Manual testing scenarios (*)

1. Check spelling is corrected
2. Check tests pass with no regressions

Contribution checklist (*)

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

[magento-engcom-team]

Hi @hugovk. 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 $VERSION instance - deploy vanilla Magento instance

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

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3304 has been created to process this Pull Request

[magento-engcom-team]

Hi @hugovk. 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.