fix documentation about use of set method
fix documentation about use of set method

Candyroot/SwiftDate,ArthurKK/SwiftDate,malcommac/SwiftDate,malcommac/SwiftDate,ArthurKK/SwiftDate,nanthi1990/SwiftDate,nanthi1990/SwiftDate,Hout/SwiftDate,malcommac/SwiftDate,Hout/SwiftDate,Candyroot/SwiftDate,Hout/SwiftDate