fix Markdown formatting in doc/API.md
fix Markdown formatting in doc/API.md

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