Fix typo

[Adamantios]

The following error was being raised from the core:

_check_matching_round_consistency
    round_to_behaviour[b.matching_round].append(b)

KeyError: <class 'packages.valory.skills.termination_abci.rounds.BackgroundRound'>

This indicates that the BackgroundRound did not have a matching behaviour specified. The reason was the typo on 1b1130e. As a result, the background_behaviours_cls attribute was never overridden to set the corresponding background behaviour.

ℹ️ Opened valory-xyz/open-autonomy#2042 to handle and show a more helpful error message on the core.