Add correct korean font file name (#1929)

correct korean font file name

Author: ShubhamOulkar

css/langs/ko.css

@@ -1,6 +1,6 @@
 /* For image callouts in writing-middleware.md */
 
-@import url('../../fonts/ko/Pretendard.css');
+@import url('../../fonts/ko/pretendard.css');
 
 html, body {font-family: Pretendard, Arial, sans-serif;}