It's nothing about "correct". 这里 is general, and 这儿 is a casual way to say 这里。This shortened casual form is developed in 河北、北京,but 儿化音 is also a part of the modern standart chinese, so it's neither "incorrect" nor a "dialect". In southern, most people don't prefer to use 儿化音,because most of southern dialects don't have this phonem. Many people don't even pronouce 翘舌音(such as sh, zh, ch, r)。 So, you can say 这里 or 这儿 as you want to, they are both "correct". If you want to sound like local people, that an other question. ;)