How can I fix this overfull hbox?

\centerline{ isn't really a latex command (it is in the latex format but just escaped from plain TeX). It makes a box \hsize wide but does not know about latex list structures and the indentation they introduce. So you line is too wide by 25pt which will be the left margin of the list item. Just remove \centerline and replace it with a \begin{center}...\end{center}.