Update chapter 6

docs/08_ko.html

docs/44_ko.html

docs/45_ko.html

docs/48_ko.html

docs/49_ko.html

docs/TOC_ko.html

docs/chapter_6_ko.html

docs/chapter_7_ko.html

frontend/lessons/ko/chapter_1.yaml

frontend/lessons/ko/chapter_5.yaml

frontend/lessons/ko/chapter_6.yaml

frontend/lessons/ko/chapter_7.yaml