Reflect companion secret character limit in example config comment

Update the comments in the example config to show that the companion secret key must be exactly 16 characters long as per https://github.com/iv-org/invidious-companion/pull/81#issuecomment-2750675405.

config/config.example.yml

@@ -90,14 +90,14 @@ db:
 ##
 ## API key for Invidious companion, used for securing the communication
 ## between Invidious and Invidious companion.
-## The size of the key needs to be more or equal to 16.
+## The key needs to be exactly 16 characters long.
 ##
 ## Note: This parameter is mandatory when Invidious companion is enabled
 ## and should be a random string.
 ## Such random string can be generated on linux with the following
 ## command: `pwgen 16 1`
 ##
-## Accepted values: a string
+## Accepted values: a string (of length 16)
 ## Default: <none>
 ##
 #invidious_companion_key: "CHANGE_ME!!"