Fix hook documentation to include error return

Fix #171

README.md

@@ -956,15 +956,16 @@ it with the `AddModelHook` method. Here is an example of a before insert hook:
 
 ```go
 // Define my hook function
-func myHook(exec boil.Executor, p *Pilot) {
+func myHook(exec boil.Executor, p *Pilot) error {
   // Do stuff
+  return nil
 }
 
 // Register my before insert hook for pilots
 models.AddPilotHook(boil.BeforeInsertHook, myHook)
 ```
 
-Your `ModelHook` will always be defined as `func(boil.Executor, *Model)`
+Your `ModelHook` will always be defined as `func(boil.Executor, *Model) error`
 
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