This is a minor typesetting error, but a potentially confusing one. It's difficult to describe here in a non-typeset way, but I'll do my best.

In the derivation for the expected out-of-sample error that is independent of any particular realization of the data set D, the final line in the derivation has a bracket size problem:

Code:

= E_x[E_D[g^(D)(x)^2] - 2E_D[g^(D)(x)]f(x) + f(x)^2]

The bracket in red in the code above is a "larger" bracket in the typesetting, and the bracket in green is a "small" bracket in the typesetting. I believe the red one should be "small" and the green one "large".