Fix missing import in i18n collection docs (#2707)

Author: delucis

docs/src/content/docs/guides/i18n.mdx

@@ -192,6 +192,7 @@ You can provide translations for additional languages you support — or overrid
 
    ```diff lang="js" ins=/, (i18nLoader|i18nSchema)/
    // src/content.config.ts
+   import { defineCollection } from 'astro:content';
    import { docsLoader, i18nLoader } from '@astrojs/starlight/loaders';
    import { docsSchema, i18nSchema } from '@astrojs/starlight/schema';