markdown: fix warnings

Author: ErikSchierboom

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

@@ -5,7 +5,6 @@ public static class Markdown
     private static string OpeningTag(string tag) => $"<{tag}>";
     private static string ClosingTag(string tag) => $"</{tag}>";
     private static string WrapInTag(this string text, string tag) => $"{OpeningTag(tag)}{text}{ClosingTag(tag)}";
-    private static bool StartsWithTag(this string text, string tag) => text.StartsWith(OpeningTag(tag));
 
     private const string HeaderMarkdown = "#";
     private const string BoldMarkdown = "__";

exercises/practice/markdown/Markdown.cs

@@ -31,7 +31,7 @@ private static string ParseText(string markdown, bool list)
         }
     }
 
-    private static string ParseHeader(string markdown, bool list, out bool inListAfter)
+    private static string? ParseHeader(string markdown, bool list, out bool inListAfter)
     {
         var count = 0;
 
@@ -68,7 +68,7 @@ private static string ParseHeader(string markdown, bool list, out bool inListAft
         }
     }
 
-    private static string ParseLineItem(string markdown, bool list, out bool inListAfter)
+    private static string? ParseLineItem(string markdown, bool list, out bool inListAfter)
     {
         if (markdown.StartsWith("*"))
         {