Docs fixes

[egeaybars123]

1-) The hyperlink for the "MIP-4" in the introduction is not found, so I updated it to the latest link of the MIP-4.
2-) In the token escrow contract, when I copied and pasted the code, it did not compile because of the fact that "async" keyword was not written for the functions. Also, in order to make the job of the developers easier, I added imports for the script (assuming that the developer installed the mina-fungible-token via npm).

[kantp]

Thanks for catching this! If you can add back the reference to the right section as per the suggestion, I'll happily merge this!

[egeaybars123]

Committed the suggested changes. Thank you! @kantp

[kantp]

Awesome, thanks @egeaybars123!