docs: replaced mitxpro-openedx-extensions with openedx-companion-auth in readme

[Anas12091101]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/4416

Description (What does it do?)

This PR replaces mitxpro-openedx-extensionswith openedx-companion-auth in the readme file. More details in the ticket

How can this be tested?

• Validate the changes in open_edx.rst and tutor.rst