Powheg: fix print for gridpack usage infos

I'm doing a small fix of the info prints introduced recently which say if a gridpack should be used or not. There is only 1 line modified, the effect is that instead of getting such kind of print:

Py:PowhegControl INFO Missing integration grid files with these patterns: ['p', 'w', 'g', 'b', 't', 'i', 'l', 'd', 'e', 'u', 'p', 'b', '', '.', 'd', 'a', 't', 'p', 'w', 'g', 'f', 'u', 'l', 'l', 'g', 'r', 'i', 'd', '', '.', 'd', 'a', 't', 'p', 'w', 'g', 'g', 'r', 'i', 'd', '', '.', 'd', 'a', 't', 'p', 'w', 'g', 'r', 'e', 'm', 'n', 'u', 'p', 'b', '', '.', 'd', 'a', 't', 'p', 'w', 'g', 'u', 'b', 'o', 'u', 'n', 'd', '*', '.', 'd', 'a', 't', 'p', 'w', 'g', 'x', 'g', 'r', 'i', 'd', '.', 'd', 'a', 't']

we would get the correct format:

Py:PowhegControl INFO Missing integration grid files with these patterns: ['pwgbtildeupb*.dat', 'pwgfullgrid*.dat', 'pwggrid*.dat', 'pwgremnupb*.dat', 'pwgubound*.dat', 'pwgxgrid.dat']

The bug was obviously due to my problems in understanding python. Sorry for the late update, I hope it's still ok for tomorrow's release.