Add Windows 11 to docs/directory_structure.md

[romw314]

Motivation / Problem

Microsoft has released Windows 11 and the documentation does not describe the directory structure for Windows 11, but it looks to be the same as on Windows 10.

Description

In docs/directory_structure.md, changed every Windows (Vista, 7, 8.1, 10) to Windows (Vista, 7, 8.1, 10, 11) (because I am playing it on Windows 11 and the structure is the same as on Windows 10).

Limitations

🟢 I don't found any limitations in this fix.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested') NO
• This PR touches english.txt or translations? Check the guidelines NO
• This PR affects the save game format? (label 'savegame upgrade') NO
• This PR affects the GS/AI API? (label 'needs review: Script API') NO
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF') NO
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[TrueBrain]

Tnx!