docs(aio): update HTTP error test example (#22844)

Update the example to match the description preceding it, which refers to the
use of the error method and ErrorEvent rather than the flush method with a
non-2xx status as shown previously.

PR Close #22844

aio/content/guide/http.md

@@ -1034,7 +1034,7 @@ Call `request.flush()` with an error message, as seen in the following example.
 
 <code-example 
   path="http/src/testing/http-client.spec.ts"
-  region="404" 
+  region="network-error"
   linenums="false">
 </code-example>