MediaUpload: Update readme to fix the name of the filter

[andrewserong]

Description

This is a tiny fix for the MediaUpload documentation to get the readme's description of the filter to match the filter that is actually used by the component here. The filter that is used is editor.MediaUpload but this part of the docs refers to it as components.MediaUpload.

How has this been tested?

Visually, check that there aren't any typos 🙂

Screenshots

Types of changes

Documentation fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

Thanks, @andrewserong.