chapter 2 fixes proposals

[teksaver]

• This chapter contains a duplicate link to the second video. The link bellow the " Kolmogorov’s Three Rules" title sends to the second video instead of the first.
• a small typo (requires/required)
• I had to wrap my head quite a lot on the "Probability mass functions" definition. The second rule reads like the actual random variables values (input) should add up to 1, instead of the pmf output.