Add JsonProperty attribute

Author: KoditkarVedant

Src/Notion.Client/Models/Database/Properties/RollupProperty.cs

@@ -26,6 +26,7 @@ public class Rollup
         [JsonProperty("rollup_property_id")]
         public string RollupPropertyId { get; set; }
 
+        [JsonProperty("function")]
         [JsonConverter(typeof(StringEnumConverter))]
         public Function Function { get; set; }
     }