docs: Fix import paths for ProxyOAuthServerProvider in README

[grimmerk]

Motivation and Context

The current README example shows incorrect import paths for the OAuth components. Updated the example to use the correct paths:

How Has This Been Tested?

This is a documentation fix only. Verified that the corrected import paths work by manually importing the components using the specified paths in a test project.

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

[grimmerk]

Kindly bump this, @ihrpr @bhosmer-ant

[ihrpr]

Thank you!