[docs] Add missing import to RTL guide

[CFarhad]

i added createtheme method to step 2 in Right-to-left guide because any new users dont know where is this method

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 568cacc

[mnajdova]

Great catch! It's a great first pull request on MUI 👌 Thank you for working on it!

[siriwatknp]

👋 The migration PR has been merged.

Please follow these steps to make sure that the contents are all updated. (Sorry for the inconvenience)

1. pull latest master from upstream to your branch
2. if your PR has changes on the *.md or demo files, you are likely to encounter conflict because all of them have been moved to the new folder.
   2.1 revert the change on those markdown files you did
   2.2 pull latest master from upstream (you should not see conflict)
   2.3 make the changes again at docs/data/material/*
3. run yarn docs:api
   • you might see the changes in docs/pages/material/api/* if your PR touches some of the components
   • it is okay if there is no changes

If you are struggle with the steps above, feel free to tag @siriwatknp