Update part3d.md

Removed validation ambiguity from exercise 3.20, fixed typo.

Author: chmromano

src/content/3/en/part3d.md

@@ -175,10 +175,10 @@ You can display the default error message returned by Mongoose, even though they
 
 #### 3.20*: Phonebook database, step8
 
-Add validation to your phonebook application, which will make sure that phone numbers are of the correct form. A phone number must
+Add validation to your phonebook application, which will make sure that phone numbers are of the correct form. A phone number must:
 
-- has length of 8 or more
-- if formed of two parts that are separated by -, the first part has two or three numbers and the second part also consists of numbers
+- have length of 8 or more
+- be formed of two parts that are separated by -, the first part has two or three numbers and the second part also consists of numbers
     - eg. 09-1234556 and 040-22334455 are valid phone numbers
     - eg. 1234556, 1-22334455 and 10-22-334455 are invalid