Updated Example 3 of the Connect-AzAccount.md page to use the correct PowerShell variable. (#22376)

In line 137 the variable created is $SecurePassword but in line 140 $SecuredPassword was being used. Copying this example leads to the following error: New-Object : Exception calling ".ctor" with "2" argument(s): "Cannot process argument because the value of argument
"password" is null. Change the value of argument "password" to a non-null value."

Author: miguel-vega

src/Accounts/Accounts/help/Connect-AzAccount.md

@@ -137,7 +137,7 @@ authenticates as a service principal.
 $SecurePassword = ConvertTo-SecureString -String "Password123!" -AsPlainText -Force
 $TenantId = 'yyyyyyyy-yyyy-yyyy-yyyy-yyyyyyy'
 $ApplicationId = 'zzzzzzzz-zzzz-zzzz-zzzz-zzzzzzzz'
-$Credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $ApplicationId, $SecuredPassword
+$Credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $ApplicationId, $SecurePassword
 Connect-AzAccount -ServicePrincipal -TenantId $TenantId -Credential $Credential
 ```