docs(MessagePayload): Correct return type of resolveFile() (#6605)

src/structures/MessagePayload.js

@@ -209,7 +209,7 @@ class MessagePayload {
   /**
    * Resolves a single file into an object sendable to the API.
    * @param {BufferResolvable|Stream|FileOptions|MessageAttachment} fileLike Something that could be resolved to a file
-   * @returns {MessageFile}
+   * @returns {Promise<MessageFile>}
    */
   static async resolveFile(fileLike) {
     let attachment;