[FTR] Fixup err msg for scripts/get_owners_for_file.js (#193581)

## Summary

Follow up of [incorrect
wording](#193277 (comment))

packages/kbn-code-owners/src/file_code_owner.ts

@@ -81,7 +81,7 @@ export async function runGetOwnersForFileCli() {
   run(
     async ({ flags, log }) => {
       const targetFile = flags.file as string;
-      if (!targetFile) throw createFlagError(`Missing --flag argument`);
+      if (!targetFile) throw createFlagError(`Missing --file argument`);
       existOrThrow(targetFile); // This call is duplicated in getPathsWithOwnersReversed(), so this is a short circuit
       const result = getCodeOwnersForFile(targetFile);
       if (result) log.success(result);