accepted format of host fingerprint should be documented

[netbrothers-tr]

The SftpConnectionProvider class accepts a host fingerprint as an constructor argument. This is documented but it lacks specifics. Did I guess right, that the fingerprint has to be provided in MD5 format? Then it would be easy to adjust the documentation like so for instance.

- 'fingerprint-string', // host fingerprint (optional, default: null),
+ 'fingerprint-string', // host MD5 fingerprint (optional, default: null),

[frankdejonge]

That's correct, PR is welcome :)