Change UserPage.ts to UserPage.tsx

[mattbal]

What docs page needs to be fixed?

https://redux.js.org/tutorials/essentials/part-6-performance-normalization#adding-user-pages

What is the problem?

The code block for UserPage is titled features/users/UserPage.ts instead of features/users/UserPage.tsx

What should be changed to fix the problem?

Change the extension to be .tsx