Fixing calculator hint to display correct definition of the constant T

Documentation and code comments for the calculator indicate that the constant T is room temperature in Kelvin, but the HTML hint that users see defines it as the freezing point of water. According to @wdickson from 3.091x, room temperature is more useful for solving chemistry problems. 

This contribution is covered by the agreement with MIT. I've added myself to the AUTHORS file. 

FYI @carsongee