docs: remove reference to client-project arguments from app-shell docs

This is no longer required because of https://github.com/angular/angular-cli/blob/493aa74019104fe029e3527b519918a0ae6febf0/packages/schematics/angular/app-shell/schema.json#L12

Author: alan-agius4

packages/schematics/angular/app-shell/app-shell-long.md

@@ -7,7 +7,7 @@ Use this command with a routing app that is accompanied by a Universal server-si
 To create an app shell, use the following command.
 
 <code-example format="." language="bash">
- ng generate app-shell --client-project my-app
+ ng generate app-shell my-app
 </code-example>
 
 * `my-app` is the name of your client application