Fix typos.

Thank you for this great book!

Another issue: On http://haskell.cs.yale.edu/cpsc-431-531/homework-assignments/, the links are broken, because there is a dash missing between the numbers in cpsc-431531:

http://haskell.cs.yale.edu/cpsc-431531/homework-assignments/assignment-1/

should be 

http://haskell.cs.yale.edu/cpsc-431-531/homework-assignments/assignment-1/

But there seems to be not content anyway :)