doc: Fix registry entry for GpgEX debug file

* README, doc/gpgex-de.html, doc/gpgex-en.html: Replace GNU\GnuPG with
Gpg4win.

get_debug_file() in main.cc does
gpgrt_w32_reg_get_string ("\\Software\\Gpg4win:GpgEX Debug File");

Before rX9c7ec74d3f79bae72da1f7f1cbeb0509f3ac136a GpgEX also looked in
\\Software\\GNU\\GnuPG.