fixes in folding

Comments to all chunks:

1. A typo: extra parenthesis breaks all folding
2. There is no need for look-behind here because ``^`` is zero-width and
   ``len(matchstr(...))`` will be same whereas ``matchstr(...)`` will trigger
   same way. (i already omitted similar look-behind in ``MarkdownLevelSA()``)
3. There could be 6 ``#`` and one space: 7 bytes altogether