Fix needed: figure 7-11 arrows should slope downward

[shaurya947]

Describe the error
To be consistent with the description of figure 7-3 (the lines are sloped down to represent elapsed time), the "commitment_signed" and "revoke_and_ack" messages lines in figure 7-11 should be sloped downwards as well. They are currently sloped upwards.

To Fix
Arrows should slope downward instead of upward.