docs: Fix commander usage in migration script

Closes #3517

docs/docs/guides/developer-guide/migrations/index.md

@@ -119,10 +119,12 @@ Running the `vendure migrate` command also uses a very similar script internally
 
 ```ts title="migration.ts"
 import { generateMigration, revertLastMigration, runMigrations } from '@vendure/core';
-import program from 'commander';
+import { Command } from 'commander';
 
 import { config } from './src/vendure-config';
 
+const program = new Command();
+
 program
     .command('generate <name>')
     .description('Generate a new migration file with the given name')