Fix highlighting html in txBlock

If html notations are in block of Perl, these are not highlighted certainly.
![https://dl.dropboxusercontent.com/u/14832699/xslate-vim/xslate-vim_old.png](https://dl.dropboxusercontent.com/u/14832699/xslate-vim/xslate-vim_old.png)

So I fixed it;
![https://dl.dropboxusercontent.com/u/14832699/xslate-vim/xslate-vim_new.png](https://dl.dropboxusercontent.com/u/14832699/xslate-vim/xslate-vim_new.png)

How do you feel?