fix 3.20

Author: mluukkai

src/content/3/en/part3d.md

@@ -189,7 +189,7 @@ Add validation to your phonebook application, which will make sure that phone nu
 
 Use a [Custom validator](https://mongoosejs.com/docs/validation.html#custom-validators) to implement the second part of the validation.
 
-If an HTTP POST request tries to add a name that is already in the phonebook, the server must respond with an appropriate status code and error message.
+If an HTTP POST request tries to add a person with an invalid phone number, the server should respond with an appropriate status code and error message.
 
 #### 3.21 Deploying the database backend to production