Make warning an actual warning (dotnet#16943)

* Make warning an actual warning

* Update angular.md

Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>

Author: ryanbrandenburg

aspnetcore/client-side/spa/angular.md

@@ -47,8 +47,9 @@ Now listening on: http://localhost:<port>
 
 Navigate to this URL in a browser.
 
-The app starts up an instance of the Angular CLI server in the background. A message similar to the following is logged: 
-*NG Live Development Server is listening on localhost:&lt;otherport&gt;, open your browser on http://localhost:&lt;otherport&gt;/*. Ignore this message&mdash;it's **not** the URL for the combined ASP.NET Core and Angular CLI app.
+> [!WARNING]
+> The app starts up an instance of the Angular CLI server in the background. A message similar to the following is logged:
+> *NG Live Development Server is listening on localhost:&lt;otherport&gt;, open a browser to http://localhost:&lt;otherport&gt;/*. Ignore this message&mdash;it's **not** the URL for the combined ASP.NET Core and Angular CLI app.
 
 ---