README: Update Guide URL

Merges: #3

Author: chrysn

README.md

@@ -5,7 +5,7 @@
 This crate contains neither actual code or documentation, but its build infrastructure is
 suitable to produce rustdoc output for all crates in the RIOT ecosystem.
 
-See the [RIOT documentation on Rust](https://doc.riot-os.org/using-rust.html) for how to use Rust on RIOT.
+See the [RIOT documentation on Rust](https://guide.riot-os.org/rust_tutorials/rust_in_riot/) for how to use Rust on RIOT.
 That page contains pointers to all the relevant components.
 
 ## Rationale for this repository's existence

src/lib.rs

@@ -7,7 +7,7 @@
 //! This is the entry point for the documentation of Rust functions for RIOT as built by rustdoc.
 //!
 //! For how to use Rust on RIOT in general, see the [RIOT documentation on
-//! Rust](https://doc.riot-os.org/using-rust.html).
+//! Rust](https://guide.riot-os.org/rust_tutorials/rust_in_riot/).
 //!
 //! Relevant crates are:
 //!