refactor: freeze StudentShort instance after initialization

lab2/student_short.rb

@@ -6,6 +6,7 @@ class StudentShort < Person
   def initialize(id:, surname_initials:, phone: nil, telegram: nil, email: nil)
     super(id: id, phone: phone, telegram: telegram, email: email)
     @surname_initials = surname_initials
+    freeze
   end
 
   def self.from_student(student)