Update custom parameter example for server builds (#389)

Author: danielchipman

docs/03-github/04-builder.mdx

@@ -229,7 +229,7 @@ There are 2 main use cases:
 
 - To pass your own custom parameters to be used with `buildMethod` above
 - To pass [Unity Build Options](https://docs.unity3d.com/ScriptReference/BuildOptions.html) (for
-  example, `customParameters: -EnableHeadlessMode` will do server builds)
+  example, `customParameters: -standaloneBuildSubtarget Server` will do server builds)
 
 _**required:** `false`_ _**default:** `""`_