Remove the leftover format placeholder from intword units

[VenishPaneliya]

intword inserts the unit without any % substitution:

singular, plural = human_powers[ordinal]
unit = _ngettext(singular, plural, math.ceil(rounded_value))
...
return f"{negative_prefix}{number} {unit}"

so a printf placeholder left in a translated unit is printed literally. The Bengali, Korean and Vietnamese catalogues still carry %d or %(value)s on the thousand…googol units:

humanize.i18n.activate("bn_BD"); humanize.intword(1_500_000_000)
humanize.i18n.activate("vi_VN"); humanize.intword(1_200_000)
humanize.i18n.activate("ko_KR"); humanize.intword(1_200_000)

On main:

'1.5 %d বিলিয়ন'
'1.2 %(value)s triệu'
'1.2 %(value)s million'

With this PR:

'1.5 বিলিয়ন'
'1.2 triệu'
'1.2 million'

This strips the placeholder and the separator that follows it (a plain space or U+00A0) and leaves the translated words untouched.

Scope is deliberately narrow: only the twelve intword power msgids are changed. Placeholders elsewhere in these catalogues are substituted normally and are left alone — naturaldelta's "%d day" and friends still need theirs. After the change, none of the 36 catalogues emits a placeholder for any of the twelve powers.

Added a parametrised regression test over the three locales, checking that intword never returns a % at any power. It fails 3/3 on main and passes here.

Two notes I did not act on, since both need a native speaker rather than a scripted edit:

• ko_KR renders several powers as the English words (million, billion), and its trillion entry reads billion.
• ko_KR and vi_VN also contain a couple of French leftovers (décillion, gogol).

Happy to split those out separately if you would like them addressed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (ce4147b) to head (0e91641).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
+ Coverage   99.56%   99.57%   +0.01%     
==========================================
  Files          12       12              
  Lines         916      938      +22     
==========================================
+ Hits          912      934      +22     
  Misses          4        4              

Flag	Coverage Δ
macos-latest	97.44% <81.81%> (-0.16%)	⬇️
ubuntu-latest	97.44% <81.81%> (-0.16%)	⬇️
windows-latest	95.30% <81.81%> (-0.11%)	⬇️

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[hugovk]

Thanks!