Touchups to Mix and OTP chapter 7

Notable changes are consistency in naming for "umbrella projects" and
"umbrella children", as discussed in #elixir-lang.
Merge pull request #297 from ernie/otp-chapter-7-touchups

Touchups to Mix and OTP chapter 7
Createad in OTP docs ch 4
Removed extra character
Merge pull request #298 from tallakt/fix_typos

Fix typos