rename cuda mode to gpu mode (#267)

## Summary

This is a documentation only change since the server has been recently
renamed. See this tweet for context
https://x.com/jeremyphoward/status/1838341110344880637

Hopefully this is OK to merge :)

README.md

@@ -29,8 +29,8 @@
             </a>
         </td>
         <td style="padding: 10px;">
-            <a href="https://discord.gg/CX2YmNmn">
-                <img src="https://dcbadge.vercel.app/api/server/cudamode?style=flat" alt="Join Our Discord">
+            <a href="https://discord.gg/gpumode">
+                <img src="https://dcbadge.vercel.app/api/server/gpumode?style=flat" alt="Join Our Discord">
             </a>
         </td>
     </tr>