Code example in libcnb-test README doesn't work

[edmorley]

After #366, this code example no longer works:
https://github.com/heroku/libcnb.rs/blob/e4e75c40816000312cfe56290a049d8df9dbb72b/libcnb-test/README.md#example

This was reported here:
#366 (comment)

We should both fix the example, and figure out why the testing of the code in that README isn't working:

libcnb.rs/libcnb-test/src/lib.rs

Lines 386 to 390 in e4e75c4

	// This runs the README.md as a doctest, ensuring the code examples in it are valid.
	// It will not be part of the final crate.
	#[cfg(doctest)]
	#[doc = include_str!("../README.md")]
	pub struct ReadmeDoctests;

(Maybe it's due to the [test] attribute used in the code example?)

[Malax]

#422 fixes that the code is incorrect. No research has been done yet why the CI check doesn't work.