fixed missing s as per Python string expression syntax

[asnov]

supervisord uses the Python string expression syntax in its configuration files, like this %(ENV_X)s
It's described here: http://supervisord.org/configuration.html#environment-variables
In the "Deploy->Supervisor->Configure" section of the documentation the final "s" were omitted:
https://docs.vapor.codes/deploy/supervisor/#configure
so copy-pasting this example was leading to wrong file names of the logs.
For example, instead of the expected file name hello-stdout.log I had the wrong hellotdout.log.

[0xTim]

Thanks!