Update README.md

mapstruct/src/main/java/com/baeldung/mapper/PersonMapper.java

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+package com.baeldung.mapper;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.factory.Mappers;
+
+import com.baeldung.dto.PersonDTO;
+import com.baeldung.entity.Person;
+
+@Mapper
+public interface PersonMapper {
+    
+    PersonMapper INSTANCE = Mappers.getMapper(PersonMapper.class);
+    
+    @Mapping(target = "id", source = "person.id", defaultExpression = "java(java.util.UUID.randomUUID().toString())")
+    PersonDTO personToPersonDTO(Person person);
+}