Update command naming to `contract-txns`.
Update command naming to `contract-txns`.

ericflo/Sia,MrStrong/Sia,mharkus/Sia,mharkus/Sia,mtlynch/Sia,NebulousLabs/Sia,NebulousLabs/Sia,MrStrong/Sia,mtlynch/Sia,ChrisSchinnerl/Sia,ChrisSchinnerl/Sia,ericflo/Sia