(cli) Typo when deleting an instance that does not exist

# Expected Result

```bash
$ stackl delete instance example
{'result': "Stack instance example can't be deleted because it does not exist"}
```

# Actual Result

```bash
$ stackl delete instance example
{'result': "Stack instance example can't be delete because it does not exist"}
```

# Steps To Reproduce

Try to delete a stack instance that does not exist