Fix typo in documentation of hmac_key

[me-kell]

The documentation under Docker-compose method (production), 2. says:

"Generate two secret keys, one for Invidious (HMAC_KEY) and one for Invidious companion (invidious_companion_key)"

• In 1st step a key for Invidious (HMAC_KEY) is generated
• In the 2nd step a key for Invidious companion (invidious_companion_key) is generated

Under 3. there is the recomended code for the docer-compose.yml file.

The instruction at the comment for hmac_key wrongly states "Use the key generated in the 2nd step".

The correct comment should be "Use the key generated in the 1st step"

[unixfox]

Hello, feel free to create a PR :). You can edit the file and create a PR. It's available at https://github.com/iv-org/documentation/edit/master/docs/installation.md