fix incorrect reference to library function

[shalzz]

While explaing the make_thread function with it's two paramters, 'pthread_create' is refered to instead of the wrapper function. In later versions of the book this was erroneuosly attempted to fix by commit d0edd0e3 but it does not make sense with the context of the previous paragraphs.

[AllenDowney]

Thank you!