docs(http): fixed example unit test for error catching (#25306)

The example unit test should test the service when the backend
application is not available, by providing a mock error response.
Although, the test will
fail as the mock response from the server is valid (it does not simulate
a
error response, but valid response with an error status 404).
This merge request fix this issue by replacing MockResponse with
MockError

This PR resolves 19499 issue

PR Close #25306

packages/http/testing/src/mock_backend.ts

@@ -176,7 +176,7 @@ export class MockConnection implements Connection {
  *        this.heroService.getHeroes()
  *            .then((heroes: String[]) => result = heroes)
  *            .catch((error: any) => catchedError = error);
- *        this.lastConnection.mockRespond(new Response(new ResponseOptions({
+ *        this.lastConnection.mockError(new Response(new ResponseOptions({
  *          status: 404,
  *          statusText: 'URL not Found',
  *        })));