Use actual class instead of singleton class in frozen error message

With the following code:

```ruby
object = []
object.singleton_class
object.freeze
object.instance_variable_set(:@a, 42)
```

The previous error message was:

```
can't modify frozen #<Class:#<Array:0x00000631d1308f78>>: []
```

With this change, the error message is:

```
can't modify frozen Array: []
```

Since we show the inspect value of the affected object, I think
including the singleton class instead of the actual class if it
exists makes the error message harder to understand.

0 files changed, 0 insertions, 0 deletions