The shape of each of the chevrons is delimited by two lateral sides whose front ends are integral with said nozzle and which, moving away from the latter, converge towards each other without joining, so that the rear ends said lateral sides are spaced apart from each other; and a curvilinear transverse line which connects the rear ends of said lateral sides forming two rounded lateral protuberances separated by a rounded intermediate recess. The lateral sides of the rafters are symmetrically inclined with respect to a longitudinal axis and the rounded lateral protuberances are offset from one another along the axis. The transverse line and the intermediate recess are dissymmetrical with respect to the axis.
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