Fix lint failures with Rust 1.70

[edmorley]

Fixes:

error: this is an outer doc comment and does not apply to the parent module or crate
 --> libcnb/src/exit_code.rs:1:1
  |
1 | ///! libcnb exit code constants
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_doc_comments
  = note: `-D clippy::suspicious-doc-comments` implied by `-D warnings`
help: use an inner doc comment to document the parent module or crate
  |
1 | //! libcnb exit code constants
  |

As seen in:
https://github.com/heroku/libcnb.rs/actions/runs/5218101938/jobs/9418580736?pr=573#step:5:231

See:
https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_doc_comments

GUS-W-13567582.

[edmorley]

Force merging since this is currently failing on main (and thus blocks things like the Dependabot PRs), is a trivial change, and everyone who might review this change is PTO or out of hours.