Fix documentation for group_sequence_provider

[giosh94mhz]

The current documentation for group_sequence_provider option in the docs is wrong.
It states that the option should be set with:

Your\Class\Name:
    group_sequence_provider: ~
    ...

But this didn't work.

By looking at the unit test I came up the the right version:

Your\Class\Name:
    group_sequence_provider: true
    ...

[wouterj]

Indeed, anything but null is allowed as a value (since they use isset() in the code). But let's use true! (it's the most logical)

thanks for investigating this bug!

[weaverryan]

Beautiful - thanks @giosh94mhz!