Update mass for Clan SRM2 improved One-Shot

[Sgage0]

Per Tactical Operations: Advanced Equipment, p. 138, improved One-Shot missile systems perform exactly as regular missile systems, except they are 0.5 tons lighter due to removal of the ammo feeds. The minimum tonnage for Inner Sphere versions is 0.5 tons, while the minimum tonnage for their Clan equivalents is 0.25 tons.
This is exactly the case with Clan SRM-2 (iOS):

mass of Clan SRM-2 (0.5 tons) - 0.5 tons (minimum 0.25 tons) ===> mass of Clan SRM-2 improved One-Shot = 0.25 tons

The current weapons data file for version of SSW (0.7.6) has the mass for SRM-2 (iOS) = 0.5 tons.