Fix Rate for DarkBoson PiPiG lines
Fixes high rates introduced in !1834 (merged)
Rate of the critical lines on 2018 data after 70k events:
|!StrippingB2KpiX2PiPiGDarkBosonLine | 0.4257| 298| 1.919| 0.535|
|!StrippingB2KX2PiPiGDarkBosonLine | 0.2643| 185| 1.211| 0.076|
Edited by Adrian Casais Vidal