Fix already-existing bug in json.kak

When a closing brace or bracket is typed, it should automatically match
indentation with it's opener. Because of an unescaped ']' literal the regexp
didn't work.

rc/filetype/json.kak

@@ -49,7 +49,7 @@ define-command -hidden json-trim-indent %{
 define-command -hidden json-indent-on-char %<
     evaluate-commands -draft -itersel %<
         # align closer token to its opener when alone on a line
-        try %< execute-keys -draft <a-h> <a-k> ^\h+[]}]$ <ret> m s \A|.\z <ret> 1<a-&> >
+        try %< execute-keys -draft <a-h> <a-k> ^\h+[\]}]$ <ret> m s \A|.\z <ret> 1<a-&> >
     >
 >