Update Rhai language reference link

[guimachiavelli]

In the update documents with function endpoint reference, the "RAI function" link points users to https://rhai.rs/book/engine/hello-world.html. @Kerollmops has indicated https://rhai.rs/book/ref/index.html would be a better resource for users.

Tasks

• Update reference/api/documents#update-documents-with-function
  • https://rhai.rs/book/ref/index.html