Update O2DPG_ROOT to O2DPG_MC_CONFIG_ROOT

Author: NicoleBastid

MC/config/PWGHF/ini/GeneratorHF_mu_ccbar_gap5_Mode2_accSmall.ini

@@ -1,7 +1,7 @@
 ### The external generator derives from GeneratorPythia8.
 [GeneratorExternal]
-fileName=${O2DPG_ROOT}/MC/config/PWGHF/external/generator/generator_pythia8_gaptriggered_hf.C
+fileName=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGHF/external/generator/generator_pythia8_gaptriggered_hf.C
 funcName=GeneratorPythia8GapTriggeredCharm(5, -4.3, -2.2)
 [GeneratorPythia8]
-config=${O2DPG_ROOT}/MC/config/PWGHF/pythia8/generator/pythia8_charm_with_mudecays_Mode2.cfg
+config=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGHF/pythia8/generator/pythia8_charm_with_mudecays_Mode2.cfg
 includePartonEvent=true