fix(examples): add private to with-shell-cmds (#9411)

### Description

Fixes #9150

yarn classic errors when using workspaces without setting private to
true

### Testing Instructions

<!--
  Give a quick description of steps to test your changes.
-->

examples/with-shell-commands/package.json

@@ -1,6 +1,7 @@
 {
   "name": "my-turborepo",
   "description": "A barebones Turborepo example for working with Task Graphs.",
+  "private": true,
   "packageManager": "[email protected]",
   "devDependencies": {
     "turbo": "^2.0.3"