Fix output directory structure builds

Fixed a bug which created multiple subdirectories with blogs containing
more than one article.

src/WebsiteBuilder.cxx

@@ -134,7 +134,8 @@ void build_blog_structure(std::string const &path, std::string const &prefix, st
 		// directories later as needed.
 		try {
 			// Create directory for the year of this article if it doesn't exist.
-			std::filesystem::path oad = obp /= year;	// Output Article Directory
+			std::filesystem::path oad = obp;
+			oad /= year;	// Output Article Directory
 			if (!std::filesystem::exists(oad)) std::filesystem::create_directory(oad);
 
 			// Do the same for the article month.