[docs] Incorrect upgrade command in v4 migration guide

[benhovinga]

Ref: http://zod.dev/v4/changelog

The upgrade command listed in the Migration Guide doesn't work.

$ npm upgrade zod@^4.0.0
npm error code EUPDATEARGS
npm error Update arguments must only contain package names, eg:
npm error     npm update zod
npm error A complete log of this run can be found in:

The npm update command only works between minor and patch versions.

Since we are going to a new major version we need to use npm install instead.

npm install zod@^4.0.0

Nathan Sebhastian made an excellent guide for updating npm packages: https://sebhastian.com/npm-update-all-packages/