Fletching Difficulty Tweak

Changed Fabrication difficulty of Sharpened Metal Arrows, Heavy Fletched Field Point Arrows, and Heavy Fletched Fire-Hardened Arrows from 5 to 4.

Previously, all components for heavy arrows were level 3, while all variants of heavy wooden and metal arrows were level 5, creating a gap in leveling progress.  This change allows the improvised point arrows to serve as a fabrication skill stepping stone.  Now archers don't have to spend level 4 grinding through unrelated recipes, but the vastly superior Heavy Wooden and Metal Arrows with real points are still sought after.