Update Get-AzKeyVaultCertificate.md - Get cert and save it as pfx section to work with PowerShell Core (#13557)

* Update Get-AzKeyVaultCertificate.md

The existing syntax does not seem to work in PowerShell Core.  Using the proposed syntax works with both PowerShell Desktop and Core.  When Calling the 'import' in this line:
$x509Cert.Import($secretByte, "", "Exportable,PersistKeySet") in PowerShell core, an error is received "Import" with "3" argument(s): "X509Certificate is immutable on this platform. Use the equivalent constructor instead."  
Declaring the cert object like this works fine:
$x509Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($secretByte, "", "Exportable,PersistKeySet")

The recommended change is a combination of the existing MS Documentation and the above which was pulled from: https://gist.github.com/holgerjay/0db1f759f93cba268d671341b787472a

* Remove duplicated line in example

Co-authored-by: Yeming Liu <[email protected]>

Author: dawsonar802

src/KeyVault/KeyVault/help/Get-AzKeyVaultCertificate.md

@@ -117,8 +117,7 @@ try {
     [System.Runtime.InteropServices.Marshal]::ZeroFreeBSTR($ssPtr)
 }
 $secretByte = [Convert]::FromBase64String($secretValueText)
-$x509Cert = new-object System.Security.Cryptography.X509Certificates.X509Certificate2
-$x509Cert.Import($secretByte, "", "Exportable,PersistKeySet")
+$x509Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($secretByte, "", "Exportable,PersistKeySet")
 $type = [System.Security.Cryptography.X509Certificates.X509ContentType]::Pfx
 $pfxFileByte = $x509Cert.Export($type, $password)