Update README with markdown badge instead of HTML

Our check for badges looks for markdown, not HTML links

Author: wesleyjellis

README.md

@@ -11,8 +11,7 @@
         <img src="https://img.shields.io/github/commit-activity/m/OpsLevel/opslevel-mcp" alt="Activity" /></a>
     <a href="https://github.com/OpsLevel/opslevel-mcp/releases">
         <img src="https://img.shields.io/github/downloads/OpsLevel/opslevel-mcp/total" alt="Downloads" /></a>
-    <a href="https://app.opslevel.com/services/opslevel_mcp/maturity-report">
-        <img src="https://img.shields.io/endpoint?style=flat&url=https%3A%2F%2Fapp.opslevel.com%2Fapi%2Fservice_level%2Fdlmj6PlFjehv6iLE6IQtEGXi_uz3LF9rA5nxb35wiY8" alt="Overall" /></a>
+    [![Overall](https://img.shields.io/endpoint?style=flat&url=https%3A%2F%2Fapp.opslevel.com%2Fapi%2Fservice_level%2Fdlmj6PlFjehv6iLE6IQtEGXi_uz3LF9rA5nxb35wiY8)](https://app.opslevel.com/services/opslevel_mcp/maturity-report)
 </p>