fix 1 month shift in computation of last recurring payment occurence

src/Entity/Payment.php

@@ -216,7 +216,12 @@ class Payment extends BasePayment
 
         $paymentEndDate = $this->getRecurrenceMonthsCount() ?
             $dateOfFirstOccurenceAfterInitialPayment->modify(
-                "+" . ($this->getRecurrenceMonthsCount() - 1) . " months" //minus one because initial payment is not considered by payzen as the first occurence
+                /* why minus 2 ?
+                 * Imagine adherent selected 2 occurences on d/m/Y, then
+                 * paymentEndDate should be d/m+1/Y,
+                 * which is already $dateOfFirstOccurenceAfterInitialPayment
+                 */
+                "+" . ($this->getRecurrenceMonthsCount() - 2) . " months"
             )
             : null; //assume no end date if recurrenceMonthsCount not set