Fix typos in the proof of Theorem 8.4.6

[EricWay1024]

In the proof of Theorem 8.4.6,

• The order of function composition g o f as well as the truncated version should be reversed;
• The identity g(t) = w' should be |g(t)|_0 = w' so that both sides lie in ||W||_0;
• |fib_f (z_0)|_0 should be ||fib_f (z_0)||_0.