Minor typo spotted

[Oakchris1955]

On chapter 3, in New trait impls, there seems to be a typo regarding a type.
The Inference breaks when a second generic impl is introduced example uses an Arc as an example, however, in the very end of the example, we have

will no longer compile, because we've previously been relying on inference to figure out the T in Box<T>.

but Box isn't mentioned anywhere.