Fix language code for english dev guide (#16563)
Fixes #16487
Summary of the issue:
currently the lang attribute for the developerGuide has the value "developerGuide": nvaccess.org/files/nvda/releases/2024.1/documentation/developerGuide.html
This is because the parent folder name is used to determine the lang code, which is true for other docs.
Description of user facing changes
Fix lang code for the developer Guide
Description of development approach
Fix lang code for the developer Guide
