[Doc] Wrong description in docs 3x mentioning 2nd generated key

[pzauner]

I noticed that in the installation guide under Docker Compose Method (Production), the configuration requires the user to manually insert the same secret key ("2nd") in three different places within the docker-compose.yml, however once the key should be a different one.

        # Use the key generated in the 2nd step 
        hmac_key: "CHANGE_ME!!"

Here the comment should be changed to 1st step / key

[unixfox]

Hello, feel free to submit a PR here: https://github.com/iv-org/documentation/edit/master/docs/installation.md