Fix capitalization in titles

[mcol]

Summary:

This is the rule @bob-carpenter suggested in #148 (comment):

• # XXX uses title punctuation for XXX (all caps other than "function" words [prepositions, determiners, auxliary verbs, other particles, etc.])

• ## XXX should use sentence punctuation for XXX (initial caps, not all caps)

Current Version:

v2.18.0

[bob-carpenter]

If we wanted to follow the example of @andrewgelman's books, everything would be in sentence punctuation. But that's one step too far for me.