Fixed scythe head icon on the 2nd manual

http://www.minecraftforum.net/topic/1659892-164tinkers-construct/page__st__6700

RichardJCox:

Second (minor): That isn't a scythe part in book 2:
Merge pull request #341 from TheVikingWarrior/patch-3

Fixed scythe head icon on the 2nd manual