fix: replace GitHub license badge with CC BY-NC-SA 4.0 badge

GitHub's license detection doesn't recognise Creative Commons licenses,
causing the automatic badge to fail. Replaced with shields.io badge
specifically for CC BY-NC-SA 4.0.

Author: Mearman

README.md

@@ -1,7 +1,7 @@
 # MCP Wayback Machine Server
 
 [![npm version](https://img.shields.io/npm/v/mcp-wayback-machine.svg)](https://www.npmjs.com/package/mcp-wayback-machine)
-[![GitHub](https://img.shields.io/github/license/Mearman/mcp-wayback-machine)](https://github.com/Mearman/mcp-wayback-machine)
+[![License: CC BY-NC-SA 4.0](https://img.shields.io/badge/License-CC%20BY--NC--SA%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by-nc-sa/4.0/)
 [![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/Mearman/mcp-wayback-machine/ci.yml?branch=main)](https://github.com/Mearman/mcp-wayback-machine/actions)
 
 An MCP (Model Context Protocol) server and CLI tool for interacting with the Internet Archive's Wayback Machine without requiring API keys.