Fix default_template_option in the sample site config

sample-site/soupault.conf

@@ -6,7 +6,7 @@
   site_dir = "site"
 
   # Where in the template the page content is inserted
-  content_selector = "#content"
+  default_content_selector = "#content"
 
   # Default page file extensions are ["html", "htm", "md", "rst"]
   # but we use .inc to demonstrate how to use something else