docs: correct example usage command

`ember app` seems to be an invalid ember-cli command, it should be `ember new`

Author: abeforgit

README.md

@@ -7,5 +7,5 @@ If you have an existing app that you would like to upgrade to use Vite consider
 ## Usage
 
 ```
-pnpm dlx ember-cli@latest app my-app-name -b @ember/app-blueprint --pnpm
+pnpm dlx ember-cli@latest new my-app-name -b @ember/app-blueprint --pnpm
 ```