Merge pull request #2244 from wuwm/master

Clarify the block comment in SDWebImageManager

@@ -186,7 +186,7 @@
                 downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
             }
             
-            // `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block bellow, so we need weak-strong again to avoid retain cycle
+            // `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block below, so we need weak-strong again to avoid retain cycle
             __weak typeof(strongOperation) weakSubOperation = strongOperation;
             strongOperation.downloadToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                 __strong typeof(weakSubOperation) strongSubOperation = weakSubOperation;