docs: Fix yarn add command (#2448)

docs/docs/guides/extending-the-admin-ui/getting-started/index.md

@@ -31,7 +31,7 @@ npm install --save-dev @vendure/ui-devkit
 <TabItem value="yarn" label="yarn">
 
 ```bash
-yarn add --save-dev @vendure/ui-devkit
+yarn add --dev @vendure/ui-devkit
 ```
 
 </TabItem>