Update .env.example

Adding the parameter INSTANCE_DISCOVER_PUBLIC="true" to prevent a HTTP 403 error at the explorer tab in the instance preview.

.env.example

@@ -9,6 +9,7 @@ ENFORCE_EMAIL_VERIFICATION="false"
 PF_MAX_USERS="1000"
 OAUTH_ENABLED="true"
 ENABLE_CONFIG_CACHE=true
+INSTANCE_DISCOVER_PUBLIC="true"
 
 # Media Configuration
 PF_OPTIMIZE_IMAGES="true"