doc: change 3 in number to three in letters (as it should be)

[guigui0246]

Then precise that it's gonna use the letter names only when using letter numbers:

print(one + two)! //three
print(1 + 2)! //3
print(1 + two)! //3 or three (50/50)
print(one + 1 + one)! //3 (1/3) or three (2/3)

[loglot]

I concur