docs(migrate): update default migration config to use production defaults (#8680)

docs/migrations.md

@@ -47,10 +47,10 @@ Before continuing further we will need to tell CLI how to connect to database. T
     dialect: 'mysql'
   },
   production: {
-    username: 'root',
-    password: null,
-    database: 'database_production',
-    host: '127.0.0.1',
+    username: process.env.PROD_DB_USERNAME,
+    password: process.env.PROD_DB_PASSWORD,
+    database: process.env.PROD_DB_NAME,
+    host: process.env.PROD_DB_HOSTNAME,
     dialect: 'mysql'
   }
 }