Fix Nix example codeblock

Author: NotNite

src/content/docs/using/install.mdx

@@ -173,7 +173,7 @@ If you use home-manager, you can import moonlight's module, and copy [your exist
 
   programs.moonlight-mod = {
     enable = true;
-    stable = {
+    configs.stable = {
       extensions = {
         allActivites.enabled = true;
         alwaysFocus.enabled = true;