Fix documentation for group_sequence_provider
bug #4145 Fix documentation for group_sequence_provider (giosh94mhz)

This PR was merged into the 2.3 branch.

Discussion
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Fix documentation for group_sequence_provider

The current documentation for `group_sequence_provider` option in the docs is wrong.
It states that the option should be set with:
```YAML
Your\Class\Name:
    group_sequence_provider: ~
    ...
````
But this didn't work.

By looking at the unit test I came up the the right version:
```YAML
Your\Class\Name:
    group_sequence_provider: true
    ...
````

Commits
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81cec5d Fix documentation for group_sequence_provider