Add descriptive book age references

src/main/resources/assets/rivencraft/lang/en_us.json

@@ -1,6 +1,7 @@
 {
   "item.rivencraft.descriptive_book": "Descriptive Book",
   "item.rivencraft.descriptive_book.unwritten": "Unwritten",
+  "item.rivencraft.descriptive_book.age": "Age %s",
   "item.rivencraft.linking_book": "Linking Book",
   "item.rivencraft.linking_book.unbound": "Unbound — sneak-use to bind",
   "item.rivencraft.linking_book.destination": "%s (%s, %s, %s)",