Update doc/contributing/erm-guidelines.md

Author: jasnell

doc/contributing/erm-guidelines.md

@@ -169,9 +169,15 @@ Or even fully anonymous objects:
 
 ```js
 function getDisposable() {
+  let disposed = false;
   return {
-    [Symbol.dispose]() {
+    dispose() {
+      if (disposed) return;
+      diposed = true;
       console.log('Resource disposed');
+    }
+    [Symbol.dispose]() {
+      this.dispose();
     },
   };
 }