Fix unordered list interpretation

Fixed a bug which caused MarkdownParser to add dashes to the beginning
of the buffer when it contains a dash which is not used to specify an
item of an unordered list.

src/MarkdownParser.cxx

@@ -230,7 +230,7 @@ std::string make_html(std::filesystem::path const &path) {
 					break;
 				case '-':
 					if (x != 0) {
-						html += c;
+						append('-');
 						break;
 					}