Chapter 4 - Typo in Plot Legend

[iamyifan]

Hey developer,

In Chapter 4, the first exercise, the given example should illustrate the posterior distribution of batting rather than the prior distribution:

posterior = prior.copy() should be added before the updating;
posterior.plot(label='posterior') instead of prior.plot(label='prior'), since we are plotting the posterior distribution.

Cheers,
Yifan