Rename RCTImageCacheDelegate -> RCTImageCache in code comment (#22406)

Summary:
Update reference to property in code comment in `RCTImageLoader`. There is no protocol named `RCTImageCacheDelegate` in the codebase. Its just `RCTImageCache` and it exists [here](f2894e58cf/Libraries/Image/RCTImageLoader.h (L22)).
Pull Request resolved: https://github.com/facebook/react-native/pull/22406

Differential Revision: D13373793

Pulled By: cpojer

fbshipit-source-id: b5edf18ebad626bf09648c079299e5acd14ba48a

Libraries/Image/RCTImageLoader.h

@@ -140,7 +140,7 @@ typedef dispatch_block_t RCTImageLoaderCancellationBlock;
 
 /**
  * Allows developers to set their own caching implementation for
- * decoded images as long as it conforms to the RCTImageCacheDelegate
+ * decoded images as long as it conforms to the RCTImageCache
  * protocol. This method should be called in bridgeDidInitializeModule.
  */
 - (void)setImageCache:(id<RCTImageCache>)cache;