Rollup merge of #80410 - MarcoIeni:patch-1, r=jonas-schievink
Rollup merge of #80410 - MarcoIeni:patch-1, r=jonas-schievink

rustdoc book: fix example

`lib.rs` file is created only if `--lib` flag is specified

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust