doc: fix broken link to README.md in protocol.md

[heiybb]

Description

Originally it links to 404, now it links back to root README.md as the client library list there.

Motivation and Context

N/A

How Has This Been Tested?

Tested OS(s): N/A

Types of changes

Documentation change (a change to documentation pages)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [x] I have included updates to all appropriate documentation.