Update docker-compose.yml

Updated for Graylog 6.3

docker-compose.yml

@@ -52,7 +52,7 @@ services:
     restart: unless-stopped
 
   graylog:
-    image: graylog/graylog:6.2
+    image: graylog/graylog:6.3
     container_name: graylog
     environment:
       # CHANGE ME (must be at least 16 characters)!