SPDY: Fix typo about length for Control bit

Found by yurui ming

Change-Id: Ibef07fa13af0de8357bdf98a038460706d0972f7
Ping-Bug: 10566
Reviewed-on: https://code.wireshark.org/review/4635
Reviewed-by: Anders Broman <a.broman58@gmail.com>
(cherry picked from commit 94d35481b773e785c3aec31f81dea4300fdb5669)
Reviewed-on: https://code.wireshark.org/review/4646
Reviewed-by: Michael Mann <mmann78@netscape.net>