added 'https://' to CDN_HOST variable example (#3446)

.env.production.sample

@@ -65,7 +65,7 @@ SMTP_FROM_ADDRESS=notifications@example.com
 # PAPERCLIP_ROOT_URL=/system
 
 # Optional asset host for multi-server setups
-# CDN_HOST=assets.example.com
+# CDN_HOST=https://assets.example.com
 
 # S3 (optional)
 # S3_ENABLED=true