Remove non-existent property value formatter from audit documentation

Co-authored-by: WeihanLi <7604648+WeihanLi@users.noreply.github.com>

Author: Copilot

docs/Usage.md

@@ -528,10 +528,7 @@ services.AddEFAutoAudit(builder =>
         .IgnoreProperty("CreatedAt")
 
         // Include unmodified properties (default is false)
-        .WithUnModifiedProperty()
-        
-        // Configure property value formatting
-        .WithPropertyValueFormatter<CustomPropertyValueFormatter>();
+        .WithUnModifiedProperty();
 });
 ```