Docs: Update CallToolResult parsing in README

[functicons]

The example in README.md for parsing the result of session.call_tool has been updated to reflect the structure of CallToolResult.

This change aligns the README example with the type definitions in mcp/types.py.

Motivation and Context

How Has This Been Tested?

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)
• [x] Documentation update

Checklist

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

Additional context