fix directive

apologies. yeah, #elseif was quite inappropriate for this case

@@ -69,7 +69,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
         _URLCallbacks = [NSMutableDictionary new];
 #ifdef SD_WEBP
         _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
-#elseif
+#else
         _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
 #endif
         _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);