accounting_l10n_fr_pca: swap 870 and 875 titles

In
https://www.associations.gouv.fr/IMG/pdf/reglt_2018-06_association.pdf (
from 2018 and applicable currently), it's written "870 dons en nature"
and "875 Bénévolat".

Eventhough in other documents from this official web site, it's still
the opposite:

 * https://www.associations.gouv.fr/IMG/pdf/plan_comptable_assoc.pdf
(from 2004) have in page 9:

> Répartition par nature de ressources
>
> 870 – Bénévolat
>
> 871 – Prestations en nature
>
> 875 – Dons en nature

 * https://www.associations.gouv.fr/IMG/pdf/benevolat_valorisation_comptable2011.pdf page 8:

> ➛ au crédit du compte « 870. Bénévolat », la contribution ;

Most of google searches yield "875 dons en nature" / "870 Bénévolat",
but let's follow this newer document.

/reviewed-on nexedi/erp5!1064