fix property to not have backing variable

Author: Mpdreamz

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -61,15 +61,10 @@ public string? NavigationTitle
 	public string FileName { get; }
 	public string Url => $"{UrlPathPrefix}/{RelativePath.Replace(".md", ".html")}";
 
-	public int NavigationIndex
-	{
-		get { return _navigationIndex; }
-		set { _navigationIndex = value; }
-	}
+	public int NavigationIndex { get; set; }
 
 	private bool _instructionsParsed;
 	private DocumentationGroup? _parent;
-	private int _navigationIndex;
 
 	public MarkdownFile[] YieldParents()
 	{