Fix path to helpers.js

src/modules/trains.ts

@@ -1,7 +1,7 @@
 import { ApiTrainsTrainByHeadcode } from '@owlboard/api-schema-types';
 import type { BaseClient, ApiResult } from '../lib/base.js';
 import { IsValidHeadcode, IsValidToc, IsValidDateStr } from '../lib/validation.js';
-import { ensureDateString } from 'src/lib/helpers.js';
+import { ensureDateString } from '../lib/helpers.js';
 import { ValidationError } from '../lib/errors.js';
 
 export class TrainsModule {