minor: include datafusion-ffi in publishing instructions (apache#13333)

Author: alamb

dev/release/README.md

@@ -281,6 +281,7 @@ Verify that the Cargo.toml in the tarball contains the correct version
 (cd datafusion/proto-common && cargo publish)
 (cd datafusion/proto && cargo publish)
 (cd datafusion/substrait && cargo publish)
+(cd datafusion/ffi && cargo publish)
 ```
 
 The CLI needs a `--no-verify` argument because `build.rs` generates source into the `src` directory.