Update generated property to let (#213)

Author: ScottPlease

Sources/StructuredQueriesCore/Documentation.docc/Articles/DefiningYourSchema.md

@@ -479,7 +479,7 @@ struct Event {
   var duration: TimeInterval
 
   @Column(generated: .stored)
-  var endAt: Date
+  let endAt: Date
 }
 ```