lib/transfer.c: Remove factor of 8 from sleep time calculation

The factor of 8 is a bytes-to-bits conversion factor, but pkt_size and
rate_bps are both in bytes. When using the rate limiting option, curl
waits 8 times too long, and then transfers very quickly until the
average rate reaches the limit. The average rate follows the limit over
time, but the actual traffic is bursty.

Thanks-to: Benjamin Gilbert