Fix grammatical errors in Chapter 4

there/their
equally as/equally
sample_size= 10000/sample_size = 10000
first iteration of A/B in chp2
edits to AB testing
fixing some text on observed frequency:
Merge pull request #90 from jvns/master

Fix some grammatical errors in Chapter 4
Update README.md
cleaning some stuff
Merge branch 'master' of github.com:CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers
edits to chp3
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Fix error in posterior graphs for text data

Label on first x-axis was wrong.
Merge pull request #92 from joehand/patch-1

Fix error in posterior graphs for text data
fixed whitespace
fix incorrect inequality
Merge pull request #95 from djv/patch-2

fix incorrect inequality
small spelling and grammar fixes