A few more typos in the tutorial #3002

Hello!

"This is very straightforword both conceptually and in implementation, but has very significant costs."

"straightforword"

"For this reason they are refered to as unique boxes."

"refered"

Named functions, like those we've seen so far, may not refer to local variables decalared outside the function - they do not "close over their environment".

"decalared"

This system has recently changed. An explanantion is forthcoming.

"explanantion"