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

rustdoc book: fix example

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

Author: m-ou-se

src/doc/rustdoc/src/what-is-rustdoc.md

@@ -10,7 +10,7 @@ CSS, and JavaScript.
 Let's give it a try! Create a new project with Cargo:
 
 ```bash
-$ cargo new docs
+$ cargo new docs --lib
 $ cd docs
 ```