server: Fix display_ptr() docs to point to `backend.handle().displa…

…y_ptr()`

This moved for the server but not the client `Backend`, and because of
not being a link nor validated in the CI this broken reference wasn't
noticed.

wayland-server/src/lib.rs

@@ -64,7 +64,7 @@
 //!
 //! - The `*mut wl_display` pointer, that you can retrieve by first retrieving the
 //! [`Backend`](crate::backend::Backend) using [`Display::backend()`], and then invoke
-//! `Backend::display_ptr().
+//! [`.handle()`][backend::Backend::handle()][`.display_ptr()`][backend::Handle::display_ptr()].
 //! - The `*mut wl_resource` pointers for the objects you need to share, by first getting the
 //! [`ObjectId`](crate::backend::ObjectId) using the [`Resource::id()`] method, and then
 //! the `ObjectId::as_ptr()` method.