Complete class names in services.yml even if no key/value

The plugin provides class name completion when the position is a key-value:

```ỳaml
services:
  Class\Name\F<caret>:
```

It should also provide completion if the position is a plain scalar, to provide completion even if the colon is missing:

```yaml
services:
  Class\Name<caret>
```
The reason why it doesnt match, is that the missing colon makes it a different psi element type.