put docs under right heading

docs/mdbook/src/rust.md

@@ -14,6 +14,10 @@ Check in the generated files under `rustutils/cpp`.
 
 We might intergrate this generation process into the build, but the tools to do so are experimental and immature as of January 2025.
 
+## Building
+
+You need to have the correct Rust toolchain(s) installed. See [Install Rust](https://www.rust-lang.org/tools/install) to install Rust.
+
 You can use `rustup` to manage toolchains. Which toolchain you needs depends on your host platform and for what platform you are trying to build. If your host and target platform are the same, you probably have the correct toolchain installed after installing Rust. For example when building for **Android** and building on a **x84 Linux** host you would use the following command:
 
 ```
@@ -22,12 +26,6 @@ rustup target add --toolchain stable-x86_64-unknown-linux-gnu aarch64-linux-andr
 
 See [Platform Support](https://doc.rust-lang.org/nightly/rustc/platform-support.html) in the Rust documentation for more details. You will get a descriptive error message when the correct toolchain is not available, so we don't list all possible combinations here.
 
-## Building
-
-You need to have the correct Rust toolchain(s) installed. See [Install Rust](https://www.rust-lang.org/tools/install) to install Rust.
-
-You can manage toolchains with rus
-
 ### CMake
 
 Set `MLN_USE_RUST=ON` when generating a configuration with CMake.