Chapter 4 - env_logger default format change causes difference from book

`env_logger` changed the default formatter to show `target` instead of `module_path` in version 0.9.0, which has caused the trace-level log showing the entering and exiting of spans to show

[.. TRACE tracing::span::active] -> Adding a new subscriber.

instead of

[.. TRACE zero2prod] -> Adding a new subscriber.

I probably should not have spent so much time finding the cause of this issue but here I am lol, might as well write it down for reference.

Start with the Chapter 4 logging example and compare its documented trace output with the output produced by env_logger 0.9. Decide whether the book should update the expected target/module_path text, then verify the revised example against the current output.

Written by the indexing model from the issue text.