docs: fix broken link to OAuthServerProvider in Authentication section of README

[samad-yar-khan]

• Updated the broken link in Authentication section of README.

Motivation and Context

• I wanted to understand how OAuth works with MCP and while browsing through the docs I found that the link to the OAuthServerProvider was broken

Screen.Recording.2025-05-07.at.6.53.36.PM.mov

How Has This Been Tested?

• Tested local branch on GH.
• Verified the file and the link is working now.

Screen.Recording.2025-05-07.at.7.05.12.PM.mov

Breaking Changes

• No breaking changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[samad-yar-khan]

@Kludex can you please take a look at this PR. Might be helpful for people trying to integrate Auth.