Use updated warning message "field is never read"

[CosineP]

this updates the book to this PR
rust-lang/rust#66469
i grepped through the rust dir to find anything and this is what came up so it's not necessarily comprehensive

[carols10cents]

Thanks for this! However, we decided to switch the output display to automatically generated output that we can update with a script. To eliminate noise because many examples have dead code, dead code is allowed. So this example no longer displays warnings.