write-out.d: Use response_code in example

[vitorgalvao]

The documentation for response_code in the manual states (emphasis mine):

The numerical response code that was found in the last transfer (formerly known as "http_code").

This suggests that response_code is preferred over http_code, yet the example uses the latter. Seeing as this code is likely to be one of the most common write-out options someone is looking for, using the newest/recommended name in the example should be more effective in getting people to use it.

[bagder]

Thanks!