Fix injection gap math

The change introduced in #1431 had the side effect that no signal events were present in recent productions using this generator, as for operator precedence one could never satisfy the condition of the if regulating the generation of signal events.

Author: mpuccio

MC/config/PWGLF/pythia8/generator_pythia8_LF.C

@@ -165,7 +165,7 @@ class GeneratorPythia8LF : public o2::eventgen::GeneratorPythia8
         if (mGapBetweenInjection > 0) {
           if (mGapBetweenInjection == 1 && mEventCounter % 2 == 0) {
             doSignal = false;
-          } else if (mEventCounter % mGapBetweenInjection + 1 != 0) {
+          } else if (mEventCounter % (mGapBetweenInjection + 1) != 0) {
             doSignal = false;
           }
         }