doc/hooks: add example for post mutation action (#2161)

* update hooks documentation to a post hook example

* Update doc/md/hooks.md

Co-authored-by: Ariel Mashraki <7413593+a8m@users.noreply.github.com>

doc/md/hooks.md

@@ -184,7 +184,10 @@ func (Card) Hooks() []ent.Hook {
 				if s, ok := m.(interface{ SetName(string) }); ok {
 					s.SetName("Boring")
 				}
-				return next.Mutate(ctx, m)
+				v, err := next.Mutate(ctx, m)
+				// Post mutation action.
+				fmt.Println("new value:", v)
+				return v, err
 			})
 		},
 	}