Quote $SERVER_FINGERPRINT to preserve newlines

Hello, I used the instruction in README.md to echo $SERVER_FINGERPRINT > ~/.ssh/known_hosts. Thanks for that tip.

However, my $SERVER_FINGERPRINT contained newlines, so I needed to quote the variable. To save others the hassle of debugging this, I'm proposing this change.

By the way, I created a similar repo https://gitlab.com/maaksoft/docker-lftp, also four months ago.