docs(core): Add lang to code sample

packages/core/src/config/custom-field/custom-field-types.ts

@@ -82,7 +82,8 @@ export type CustomFieldConfig =
  * * `validate?: (value: any) => string | LocalizedString[] | void`: A custom validation function.
  *
  * The `LocalizedString` type looks like this:
- * ```
+ *
+ * ```TypeScript
  * type LocalizedString = {
  *   languageCode: LanguageCode;
  *   value: string;