Add newline to `PublicKey::write_openssh_file` output #414

Job	Run time
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	45s
build (x86_64-apple-darwin, macos-latest, stable)	1m 22s
no_std (1.65.0, thumbv7em-none-eabi)	9m 58s
test (1.65.0)	26m 34s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 52s
no_std (1.65.0, wasm32-unknown-unknown)	9m 47s
test (stable)	21m 18s
no_std (stable, thumbv7em-none-eabi)	6m 59s
no_std (stable, wasm32-unknown-unknown)	7m 11s
minimal-versions / minimal-versions	45s
	1h 26m 31s

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