Prettier Image loader code (facebook#23701)

Summary:
We use indent width with 2 spaces, but `RCTImageLoader` use 4 spaces, it's hard to code, code one line, adjust indent, or change Xcode's preference temporary. So let's prettier it.

[iOS] [Fixed] - Prettier Image loader code
Pull Request resolved: facebook#23701

Differential Revision: D14278082

Pulled By: cpojer

fbshipit-source-id: 6e7db50741b0981f45622c19981cb5fd9bbc0a97

Author: zhongwuzw