markdown: fix

Author: ErikSchierboom

exercises/practice/markdown/.meta/Example.cs

@@ -39,7 +39,7 @@ private static string ParseText(this string markdown, bool list)
         return list ? textHtml : textHtml.WrapInTag(ParagraphTag);
     }
 
-    private static Tuple<bool, string> ParseHeader(this string markdown, bool list)
+    private static Tuple<bool, string>? ParseHeader(this string markdown, bool list)
     {
         var headerNumber =
             markdown
@@ -56,7 +56,7 @@ private static Tuple<bool, string> ParseHeader(this string markdown, bool list)
         return Tuple.Create(false, html);
     }
 
-    private static Tuple<bool, string> ParseLineItem(this string markdown, bool list)
+    private static Tuple<bool, string>? ParseLineItem(this string markdown, bool list)
     {
         if (!markdown.StartsWith(ListItemMarkdown))
             return null;
@@ -105,4 +105,4 @@ public static string Parse(string markdown)
 
         return list ? html + ClosingTag(ListTag) : html;
     }
-}
+}