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

[peter-lyons-kehl]

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.)