Fix STR_SCENARIO_START_ID to allow WC_II_custom_terrain modification

[Adriankhl]

In utils/custom_terrain_mod.cfg there is a allow_scenario= {STR_CAMPAIGN_WC_II_SCENARIOS_ID}

However, the ID is defined as World_Conquest_II_{PLAYERS_NUMBER}p in utils/strings/scenario.cfg,
where it should be WC_II_{PLAYERS}p in scenarios/WC_II_scenario.cfg

[Adriankhl]

I guess I should comment and modify the coming mainline version instead of this one.

[gfgtdf]

for the mainline version and most likely for the 1.15 aswell i will remove the custom terrain mod, since most of the changes it did were not yet mainline quality in my opinion.