Ch 5: First mention of 1.96

[mine-cetinkaya-rundel]

How about adding a bit more info here. Instead of just

https://github.com/OpenIntroOrg/openintro-statistics/blob/b7e661929a25ca0f7fb530139e1c5aa015f1f459/ch_foundations_for_inf/TeX/ch_foundations_for_inf.tex#L732-L736

something like

While about 95\% of the data is within 2 standard deviations 
in a normal distribution, it would be more precise to use 
a value of 1.96 standard deviations as the area under the normal 
curve within 1.96 standard deviations is closer to 0.95. This more 
precise value is typically what is used to construct a 95\% confidence 
interval.

[OpenIntroOrg]

Sounds good -- now updated.