Update group_member.md

Makes the code in "example usage" reflect the actual properties that are output by azuread_grouup and azuread_user;

docs/resources/group_member.md

@@ -32,8 +32,8 @@ resource "azuread_group" "example" {
 }
 
 resource "azuread_group_member" "example" {
-  group_object_id  = azuread_group.example.id
-  member_object_id = data.azuread_user.example.id
+  group_object_id  = azuread_group.example.object_id
+  member_object_id = data.azuread_user.example.object_id
 }
 ```