Update Semgrep link in README.md

[emjin]

Description

Update the link for the Semgrep MCP server to point to the semgrep/semgrep repo, where it now lives.

Server Details

• Server:
• Changes to:

Motivation and Context

We moved the MCP server from its dedicated repo to our semgrep repo

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)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context