Merge pull request #217 from martinxyz/master

separation_plot.py (Chapter2): fix plot off-by-one problem
Merge pull request #218 from martinxyz/master

separation_plot.py: fix expected value bar
Fix minor spelling errors
Merge pull request #220 from ugriz/master

Fix minor spelling errors
fix code in comment
Merge pull request #222 from heinzK1X/master

fix code in comment
changed slightly confusing phrase
correct minor typo
Merge pull request #223 from RoyalTS/master

changed slightly confusing phrase
Merge pull request #1 from CamDavidsonPilon/master

new updates