Chapter 03 - Correction when explaining ppf

[Lucas-Okamura]

There is an issue on chapter 03, in the following paragraphs "(...) saying that 50% of the mass of the standard normal distribution is below 0.0." and "(...) In other words, 0.05% of the mass falls above this value."

It should be "(...) saying that 50% of the mass of the standard normal distribution is below the sum between the mean and 0.0 times standard deviation." and "(...) In other words, 0.5% of the mass falls above this value.", respectively.

[donelianc]

I made PR #257 to fix the second suggestion from @Lucas-Okamura's comment.

I left the first suggestion to @matheusfacure since it looks like a wording issue.

But agree with Lucas since the standard normal distribution doesn't imply that the distribution's mean is 0 (although it's pretty common to use a distribution with mean=0 to facilitate calculations and examples maybe that's why Matheus wrote "below 0.0").