Removed non-existent item from the ToC in the theming doc (#2674)

Removed link link in table of contents to non-existent Usage section

Author: dylandbl

docs/theming.mdx

@@ -9,7 +9,6 @@ Add `ThemeProvider` to the top level of your app and access the theme with `prop
 ## Table of Contents
 
 - [Examples](#examples)
-- [Usage](#usage)
 - [API](#api)
   - [ThemeProvider](#themeprovider-reactcomponenttype)
   - [withTheme](#withthemecomponent-reactcomponenttype-reactcomponenttype)