doc: Improve sftp_read_sync() example.
doc: Improve sftp_read_sync() example.

mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh