fix "verify firefly" on windows

[huhn511]

use powershell

1. Open powershell
2. Verify the firefly-desktop-1.0.0.exe file's signature.

Get-AuthenticodeSignature -FilePath C:\Users\yourname\Downloads\firefly-desktop-1.0.0.exe | Format-List

3. Make sure that the following information matches the output of the command:

[Subject] CN=IOTA Stiftung, O=IOTA Stiftung, L=Berlin, C=DE
[Issuer] CN=DigiCert SHA2 Assured ID Code Signing CA, OU=www.digicert.com, O=DigiCert Inc, C=US
[Serial Number] 017E5337DECA4C60BD1881E2097EBD3B