Fix wrong variable name and adjust path

Author: Phippe

docs/guides/distribution/sign-windows.md

@@ -55,7 +55,7 @@ We now need to import our `.pfx` file.
 
 1. Assign your export password to a variable using `$WINDOWS_PFX_PASSWORD = 'MYPASSWORD'`
 
-2. Now Import the certificate using `Import-PfxCertificate -FilePath Certs/certificate.pfx -CertStoreLocation Cert:\CurrentUser\My -Password (ConvertTo-SecureString -String $env:WINDOWS_PFX_PASSWORD -Force -AsPlainText)`
+2. Now Import the certificate using `Import-PfxCertificate -FilePath certificate.pfx -CertStoreLocation Cert:\CurrentUser\My -Password (ConvertTo-SecureString -String $WINDOWS_PFX_PASSWORD -Force -AsPlainText)`
 
 ### C. Prepare Variables