chapter 6 should reference :person/born

The following code for exercise 1 is wrong and I had to cheat (it should use `:person/born`)

```clojure
[:find ?name
 :in $ ?age ?today
 :where
 [?p :person/name ?name] 
 [?p :person/birth ?birth]
 [(tutorial.fns/age ?birth ?today) ?age]]
```

However the example code in chapter 6 references `:person/birth`

```clojure
[:find ?age
 :in $ ?person ?today
 :where
 [?p :person/name ?name]
 [?p :person/birth ?birth]
 [(tutorial.fns/age ?birth ?today) ?age]]
```