fix typo in Attachment docs (#2885)

Author: fgardt

src/model/channel/attachment.rs

@@ -38,7 +38,7 @@ pub struct Attachment {
     /// The filename of the file that was uploaded. This is equivalent to what the uploader had
     /// their file named.
     pub filename: String,
-    /// Sescription for the file (max 1024 characters).
+    /// Description for the file (max 1024 characters).
     pub description: Option<String>,
     /// If the attachment is an image, then the height of the image is provided.
     pub height: Option<u32>,