Fixed typo introduced on PR #4308

The context being generated for triggering save events is currently malformed due to a un-noticed typo introduced on our PR #4308.

The context is currently being generated using:

```php
$context    = $this->option . '_' . $this->name;
````

instead of:

```php
$context    = $this->option . '.' . $this->name;
```

This PR solves this problem.

I'm really sorry for the trouble.

Thank you for taking the time to consider this PR.