[Etherpad] Fix Compose volume name

pica-etherpad/docker-compose.yml

 version : "2.4"
 
 volumes:
-  etherpad-db-volume:
+  etherpad-db:
     external: true
-    name: "etherpad-db"
 
 networks:
   docker_default:
@@ -48,7 +47,7 @@ services:
     cpus: "0.40"
     pids_limit: 1024
     volumes:
-      - etherpad-db-volume:/var/lib/mysql
+      - etherpad-db:/var/lib/mysql
     env_file: ./secrets/etherpad-db.secrets
     restart: unless-stopped
     # See https://stackoverflow.com/questions/41134785/how-to-solve-mysql-warning-innodb-page-cleaner-1000ms-intended-loop-took-xxx