message: docs(aio): update HTTP error test example
  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
diff: @@ -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>
  