Yes, you are right, there is a typo. Clearly, if we have a sum like sum_{i=1}^n (a_i+ell) then that is equal to (sum_{i=1}^n a_i) + (sum_{i=1}^n ell) which is equal to (sum_{i=1}^n a_i) + n*ell.

It has been corrected now. Thanks for pointing this out!