docs(ffi): document nightly requirement for FFI

Update the command to use `--crate-type`.

capi/README.md

@@ -10,8 +10,8 @@ Because of that, it's only accessible if `--cfg hyper_unstable_ffi` is passed to
 
 ## Building
 
-The C API is part of the Rust library, but isn't compiled by default. Using `cargo`, it can be compiled with the following command:
+The C API is part of the Rust library, but isn't compiled by default. Using a nightly release of `cargo`, starting with `nightly-2022-03-02`, it can be compiled with the following command:
 
 ```
-RUSTFLAGS="--cfg hyper_unstable_ffi" cargo build --features client,http1,http2,ffi
+RUSTFLAGS="--cfg hyper_unstable_ffi" cargo +nightly rustc --features client,http1,http2,ffi -Z unstable-options --crate-type cdylib
 ```