[BUG] Fix multirocket bug variables not updated in loop

[angus924]

Update variables combination_index and n_channels_start in loop for first order difference.

Reference Issues/PRs

Fixes #2967

What does this implement/fix? Explain your changes.

Ensures variables combination_index and n_channels_start are updated in loop for function _transform_multi for the first order difference.

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[TonyBagnall]

thanks @angus924