config: Fix syntax highlighting setting

Apparently, Zola moved  the `highlight_code` and `highlight_theme`
settings into a new `markdown` section.

config.toml

@@ -4,16 +4,17 @@ base_url = "https://dustin.hatch.name/"
 # Whether to automatically compile all Sass files in the sass directory
 compile_sass = true
 
-# Whether to do syntax highlighting
-# Theme can be customised by setting the `highlight_theme` variable to a theme supported by Zola
-highlight_code = true
-highlight_theme = 'axar'
-
 # Whether to build a search index to be used later on by a JavaScript library
 build_search_index = false
 
 generate_feed = true
 feed_filename = 'atom.xml'
 
+[markdown]
+# Whether to do syntax highlighting
+# Theme can be customised by setting the `highlight_theme` variable to a theme supported by Zola
+highlight_code = true
+highlight_theme = 'axar'
+
 [extra]
 # Put all your custom variables here