fix(docs): proper postinstall script file name

I think this change was incorrect: #2024

The point of this example is that the same script is being used for two different stages (`install` and `post-install`) so it would be a good idea to look at the `npm_lifecycle_event` environment variable inside this script to determine which stage is being run.

PR-URL: #3282
Credit: @KevinFCormier
Close: #3282
Reviewed-by: @wraithgar

docs/content/using-npm/scripts.md

@@ -304,7 +304,7 @@ For example, if your package.json contains this:
 {
   "scripts" : {
     "install" : "scripts/install.js",
-    "postinstall" : "scripts/postinstall.js",
+    "postinstall" : "scripts/install.js",
     "uninstall" : "scripts/uninstall.js"
   }
 }