Typo in Tracing rule section

[m-clark]

• "A and C" should be "B and C" in the prose.
• The formula "ac + ab * ac" should be "bc + ba * ac" (the direct path is B→C = bc, not A→C = ac; the indirect path goes backward through the B←A arrow, then forward along A→C).

For completeness, the A–C tracing would be: ac + ab * bc = 0.167 + 0.200 × 0.667 = 0.167 + 0.133 = 0.300 ✓, which matches the matrix.

Thanks to Matt Thomas at the Cornell Statistical Consulting Unit for catching this.