corrects VNET resource varibale name used in example (hashicorp#19641)

Co-authored-by: studentN <[email protected]>

website/docs/r/virtual_machine.html.markdown

@@ -40,7 +40,7 @@ resource "azurerm_virtual_network" "main" {
 resource "azurerm_subnet" "internal" {
   name                 = "internal"
   resource_group_name  = azurerm_resource_group.example.name
-  virtual_network_name = azurerm_virtual_network.example.name
+  virtual_network_name = azurerm_virtual_network.main.name
   address_prefixes     = ["10.0.2.0/24"]
 }