fix Heroku env var instructions

[philippbosch]

The extra space character between FATHOM_SECRET and $(openssl …) broke the command because it set FATHOM SECRET to '' and interpreted the generated secret as the name of the next env var to set.

$ heroku config:set PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/app/bin \
  FATHOM_DATABASE_DRIVER=postgres \
  FATHOM_DATABASE_URL=$(heroku config:get DATABASE_URL) \
  FATHOM_DEBUG=true \
  FATHOM_SECRET= $(openssl rand -base64 32) \
  FATHOM_GZIP=true
Setting PATH, FATHOM_DATABASE_DRIVER, FATHOM_DATABASE_URL, FATHOM_DEBUG, FATHOM_SECRET, 0ACl9FJMKgllHAXZUUqmm7PQ3P92MuQlN6lt/5OLdkg, FATHOM_GZIP and restarting ⬢ my-app-name... !
 ▸    Name is invalid

(I'm not using the secret from this snippet anywhere)

[philippbosch]

Oops. Duplicate of #279. Closing.