Chapter 9: Add example for iterating through a list of maps

[radar]

Having an example like:

people = [%{"name" => "The Author", "age" => "30"}, %{"name" => "The Reader", "age" => "40"}]
Enum.map(people, fn (person) -> person["name"] end)

Would be an excellent example to use in Chapter 9 when demonstrating Enum.map with maps. Might be able to show the shorter ampersand syntax here too:

Enum.map(people, &(&1["name"]))