Remove reference to `Secret` and `Access Tokens` in readme file

[tomasr78]

The secrets are deprecated, but the documentation still references them. Remove.

https://github.com/ConvertAPI/convertapi-cli/blob/main/README.md

[tomasr78]

Also, there is a reference to access tokens, which is also deprecated https://www.convertapi.com/a/access-tokens

Now we use the API Token from https://www.convertapi.com/a/authentication

[paulius-petkus]

Done.