Fix TemplateSyntaxError in page.html jinja template

```
File "/srv/pelican/themes/pelican-clean-blog/templates/page.html", line 7, in templat
<header class="intro-header" style="background-image: url('{{ page.header_cover|default({{ SITEURL }}/{{ THEME_STATIC_DIR }}/images/home-bg.jpg') }})">
jinja2.exceptions.TemplateSyntaxError: expected token ':', got '}'
```