chore: fix custom code comment to not trigger clippy

[frederikrothenberger]

Having a doc comment in the auto-generated file triggers clippy:

warning: empty line after doc comment
    --> /Users/frederikrothenberger/repos/one-core/target/debug/build/core-server-0e480849ff4b80bc/out/shadow.rs:2287:1
     |
2287 | / /// Below code generated by project custom from by build.rs
2288 | |
     | |_^
2289 |   #[allow(dead_code)] pub const APP_VERSION: Option<&str> = None;
     |                       ----------------------------------- the comment documents this constant
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
     = note: `#[warn(clippy::empty_line_after_doc_comments)]` on by default
     = help: if the empty line is unintentional remove it

Simply changing the comment to be a regular one (with just //) fixes the issue.