wc: adapt error message to pass wc-files0-from

[cakebaker]

This PR is similar to #8748 and should make the GNU test wc-files0-from pass again.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/wc/wc-files0-from is no longer failing!

[codspeed-hq]

CodSpeed Performance Report

Merging #8749 will not alter performance

_{Comparing cakebaker:wc_fix_files0_from (905714c) with main (20c2779)}

Summary

✅ 93 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩