Fix rss.xml template

I had text like this in my post:

```
>>>> d = {'a': 1, 'b': 2, 'c': 3}
>>> da = d['a']
>>> da
```

But when the rss.xml was generated, the '>>>' characters were simple copied to the output and there was a complaint about invalid characters in browser. So I added JINJA2 tag "| escape" to the template which escapes invalid characters (&, <,  >, ...) to HTML entities.