[docs] Fix migration guide upgrade command (#5021)

* Fix upgrade command

* Fix upgrade command

* Fix upgrade command

Author: benhovinga

packages/docs/content/v4/changelog.mdx

@@ -11,7 +11,7 @@ This migration guide aims to list the breaking changes in Zod 4 in order of high
 {/* To give the ecosystem time to migrate, Zod 4 will initially be published alongside Zod v3.25. To use Zod 4, upgrade to `zod@3.25.0` or later: */}
 
 ```
-npm upgrade zod@^4.0.0
+npm install zod@^4.0.0
 ```
 
 {/* Zod 4 is available at the `"/v4"` subpath:

packages/docs/content/v4/index.mdx

@@ -24,7 +24,7 @@ Huge thanks to [Clerk](https://go.clerk.com/zod-clerk), who supported my work on
 To upgrade: 
 
 ```
-npm upgrade zod@^4.0.0
+npm install zod@^4.0.0
 ```
 
 

packages/docs/content/v4/versioning.mdx

@@ -14,7 +14,7 @@ import { Callout } from "fumadocs-ui/components/callout";
 To upgrade to Zod 4:
 
 ```
-npm upgrade zod@^4.0.0
+npm install zod@^4.0.0
 ```
 
 If you are using Zod 4, your existing imports (`"zod/v4"` and `"zod/v4-mini"`) will continue to work forever. However, after upgrading, you can *optionally* rewrite your imports as follows: