typo fix

src/Discord.Net.Core/Extensions/AttachmentExtensions.cs

@@ -3,7 +3,7 @@ namespace Discord
     public static class AttachmentExtensions
     {
         /// <summary>
-        ///     Gets wheter the message's attachments are spoilers or not.
+        ///     Gets whether the message's attachments are spoilers or not.
         /// </summary>
         public static bool IsSpoiler(this IAttachment attachment)
             => attachment.Filename.StartsWith(IAttachment.SpoilerPrefix);