Docs: Removes duplicate output from FFI example

The "Rust Inside Other Languages" page includes a library example. The
reference specifies printing "done!" when the code finishes running, and
the language examples (Ruby, Python, JS) all do this in their code.
However, the Rust library example code *also* does this, so that the
examples as written would output "done!" twice.

This removes the "done!" from the Rust example code to clarify the docs.

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