Fix option in syntax highlighting docs
The sentence mentions `highlighting_themes_css` but the actual option is `highlight_themes_css`.
@@ -227,7 +227,7 @@ you get CSS class definitions, instead.
</pre>
```
-Zola can output a css file for a theme in the `static` directory using the `highlighting_themes_css` option.
+Zola can output a css file for a theme in the `static` directory using the `highlight_themes_css` option.
```toml
highlight_themes_css = [
