Fix copypasta function documentation in ImageLoaderModule (#38029)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38029

# Changelog:
[Internal] -

A trivial fix - I was looking into some deeper issues with `Image.prefetch`, noticed that code docs were copypasted and incorrect.

Reviewed By: cipolleschi

Differential Revision: D46959350

fbshipit-source-id: 4e4196f9399d91402d9b206c20f2bbd0d1d0eb2a