Docs: Change README to correct pip installation command for MCP CLI support

[codingjenny]

Motivation and Context

When I followed the installation instructions in the README and used pip install mcp, it did install something. However, when I ran mcp dev server.py, I encountered the following error:

Error: typer is required. Install with 'pip install mcp[cli]'

I then removed the previous installation and tried pip install mcp[cli], and it finally worked.

How Has This Been Tested?

I only modified the README, so I just ran the tests mentioned in CONTRIBUTING.md, and they all passed successfully.

Breaking Changes

No

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

[Kludex]

Thanks!