Fixed TemplateSyntaxError when using jinja-default template

The `jinja-default` template has a missing `%}` in `post.tmpl` - which causes a
`TemplateSyntaxError: expected token 'end of statement block', got 'href'` when doing a build using this theme.

There's also a missing endblock on line 6 which I just found, which wasn't in the version of this theme from 5.4.4, where I found the other issue.

This fixes them, allowing you to actually use that theme.