fix: Update mintNft API using proper format

[johnedvard]

Summary

• Update mintNft API using proper format
• Update mint_nft example in README

I came across this StackOverflow question, and started debugging. It looks like the current /mint_nft-API doesn't add receiver_id to the functionCall, and is using the incorrect name and format for metadata. This PR addresses this. I hope this PR can be helpful. The current way of calling functionCall is deprecated, but this small change will hopefully make the current /mint_nft example work.

I ran the server and tested the API using curl command:

curl --header "Content-Type: application/json" -d '{"token_id": "example_id", "token_metadata": {"media":"https://ipfs.io/ipfs/Qme7ss3ARVgxv6rXqVPiikMJ8u2NLgmgszg13pYrDKEoiu"},"account_id": "example.testnet","private_key": "somePrivateKey","contract": "example.testnet"}' -X POST http://localhost:3000/mint_nft