* fix two typos that keep an example from a) compiling and b) making sense.
  Taken together these changes produce an example which can be used to do
  what the cookbook claims it'll do.

  * removed an extraneous '{'.
  * replaced a "$class" (which isn't mentioned anywhere else in the example)
    with a "$pkg" (which is).
* a bit of wordsmithing to better describe the example's goal and
  behaviour.