Fix typo in error enum

Sources/ImageDownloader.swift

@@ -90,7 +90,7 @@ public enum KingfisherError: Int {
     case invalidURL = 20000
     
     /// The downloading task is cancelled before started.
-    case downloadCanelledBeforeStarting = 30000
+    case downloadCancelledBeforeStarting = 30000
 }
 
 /// Key will be used in the `userInfo` of `.invalidStatusCode`
@@ -292,7 +292,7 @@ extension ImageDownloader {
 
         if let modifier = options?.modifier {
             guard let r = modifier.modified(for: request) else {
-                completionHandler?(nil, NSError(domain: KingfisherErrorDomain, code: KingfisherError.downloadCanelledBeforeStarting.rawValue, userInfo: nil), nil, nil)
+                completionHandler?(nil, NSError(domain: KingfisherErrorDomain, code: KingfisherError.downloadCancelledBeforeStarting.rawValue, userInfo: nil), nil, nil)
                 return nil
             }
             request = r