Fix spelling mistake in manpage.

Fixes #610, pointed out by @borenius-dfn-de.

src/iperf3.1

@@ -76,7 +76,7 @@ There will be at least one line of output per measurement interval (by
 default a measurement interval lasts for one second, but this can be
 changed by the -i option).
 Each line of output includes (at least) the time since the start of
-the test, amount of data transfered during the interval, and the
+the test, amount of data transferred during the interval, and the
 average bitrate over that interval.
 Note that the values for each measurement interval are taken from the
 point of view of the endpoint process emitting that output (in other