Minor touchups to Mix and OTP chapter 6.

An important one is removing the potentially-confusing use of the word
"global" when referring to table names. While they are "global" in the
sense I, as a Rubyist, might think of "global", they are "local" in
terms of Erlang name registration, and other nodes can't see them.
Merge pull request #293 from ernie/otp-chapter-6-touchups

Minor touchups to Mix and OTP chapter 6.