Update the Functions example README to use the new .env.enc

[dwightjl]

https://github.com/smartcontractkit/smart-contract-examples/blob/main/add-functions-to-project/README.md

The readme still refers to using the .env file. It should now use .env.enc as found in the new docs. smartcontractkit/documentation#1256 (review)

[buddhiraz]

Can I work on this ?