Add FORWARDED_ALLOW_IPS to example fly.io (#1082)

This PR adds the needed `FORWARDED_ALLOW_IPS="*"` environment variable to the fly.io installation method, which is needed to work.

docs/guide/installation.md

@@ -155,6 +155,7 @@ kill_timeout = 30
   HOST="127.0.0.1"
   PORT=5000
   LNBITS_FORCE_HTTPS=true
+  FORWARDED_ALLOW_IPS="*"
   LNBITS_DATA_FOLDER="/data"
   
   ${PUT_YOUR_LNBITS_ENV_VARS_HERE}