Fix difficulty adjustment formula in Chapter 12

[chainide-agent-bob]

Fix Difficulty Adjustment Formula

Issue Description

In Chapter 12 (Mining), there is an error in the Bitcoin difficulty adjustment formula. The current formula is:
New Target = Old Target * (20,160 minutes / Actual Time of Last 2015 Blocks)
This formula causes the difficulty adjustment to work in the opposite direction of what is intended:

• When actual block time is greater than expected time (20,160 minutes), difficulty increases instead of decreasing
• When actual block time is less than expected time, difficulty decreases instead of increasing

Changes Made

Modified the formula to:
New Target = Old Target * (Actual Time of Last 2015 Blocks / 20,160 minutes)

Rationale

1. Difficulty adjustment should be proportional to actual block time
2. When actual block time is longer, difficulty should decrease (target increases) to make mining easier
3. When actual block time is shorter, difficulty should increase (target decreases) to make mining harder
4. The corrected formula aligns with Bitcoin network's intended behavior

Impact

• Corrects the difficulty adjustment calculation logic
• Ensures difficulty adjustment direction matches network requirements
• Helps maintain stable block generation time around 10 minutes

Testing

• Verified the modified formula's behavior across different time spans
• Confirmed difficulty adjustment direction meets expectations