Fix incorrect PostgreSQL data path

`postgresql` Docker image stores the database files in `/var/lib/postgresql/data`.

Signed-off-by: Igor Liška<igor.liska@panaxeo.com>

docs/3.0.0-beta.x/installation/docker.md

@@ -55,7 +55,7 @@ services:
       POSTGRES_USER: strapi
       POSTGRES_PASSWORD: strapi
     volumes:
-      - ./data:/data/postgres
+      - ./data:/var/lib/postgresql/data
     ports:
       - '5432:5432'
 ```