fix RichTextPropertyItem parsing

Author: KoditkarVedant

Src/Notion.Client/Models/PropertyItems/RichTextPropertyItem.cs

@@ -1,9 +1,12 @@
-namespace Notion.Client
+using Newtonsoft.Json;
+
+namespace Notion.Client
 {
     public class RichTextPropertyItem : SimplePropertyItem
     {
         public override string Type => "rich_text";
 
+        [JsonProperty("rich_text")]
         public RichTextBase RichText { get; set; }
     }
 }