selftests: mptcp: fix mibit vs mbit mix up

The estimated time was supposing the rate was expressed in mibit
(bit * 1024^2) but it is in mbit (bit * 1000^2).

This makes the threshold higher but in a more realistic way to avoid
false positives reported by CI instances.

Before this patch, the thresholds were at 7561/4005ms and now they are
at 7906/4178ms.

While at it, also fix a typo in the linked comment, spotted by Mat.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/310


Fixes: 1a418cb8 ("mptcp: simult flow self-tests")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>