another meta.so -> meta.se link update

Markdown.Editor.js

@@ -1899,7 +1899,7 @@
         //
         // Since this is essentially a backwards-moving regex, it's susceptible to
         // catstrophic backtracking and can cause the browser to hang;
-        // see e.g. http://meta.stackoverflow.com/questions/9807.
+        // see e.g. http://meta.stackexchange.com/questions/9807.
         //
         // Hence we replaced this by a simple state machine that just goes through the
         // lines and checks for a), b), and c).