Rustdoc Book: replace `missing_doc_code_examples` with `rustdoc::missing_doc_code_examples`

### Location

src/doc/rustdoc/src/write-documentation/what-to-include.md

### Summary

Replace `missing_doc_code_examples` -> `rustdoc::missing_doc_code_examples`. (I'm providing a PR.)

(A related issue: #109601.)