Update layers.idx file example in docs

[scottfrederick]

The documentation for 2.3.0 includes an example of a layers.idx file, but the example doesn't match the format we ended up with for the release.

The example there should be updated, and we should consider providing a more formal description of the file format instead of just an example.

[scottfrederick]

we should consider providing a more formal description of the file format instead of just an example

Ignore that part, it exists here: https://docs.spring.io/spring-boot/docs/2.3.0.RELEASE/reference/htmlsingle/#executable-jar-war-index-files-layers