Update public/js/index.js

simplify the regex
suggested by yukaii in #1605

Co-authored-by: Yukai Huang <yukaihuangtw@gmail.com>

Author: tamo

public/js/index.js

@@ -3137,7 +3137,7 @@ function checkInContainerSyntax () {
 
 function matchInContainer (text) {
   var match
-  match = text.match(/(^|\n):::/gm)
+  match = text.match(/^:::/gm)
   if (match && match.length % 2) {
     return true
   } else {