Update nft-tokens.mdx

[raduchis]

Description of the pull request (what is new / what has changed)

Did you test the changes locally ?

• [ ] yes
• [x] no

Which category (categories) does this pull request belong to?

• [ ] document new feature
• [ ] update documentation that is not relevant anymore
• [ ] add examples or more information about a component
• [x] fix grammar issues
• [ ] other