play_controller::check_victory(): use underscore in event name

Semantically this doesn't change anything because spaces in event names are
automatically replaced with underscores.

I just spent a considerable amount of time searching for code that fires
the enemies_defeated event, only to find that it's called
"enemies defeated", with a space, at the call site. Let's use the
underscored name everywhere to make the function easier to find.

src/play_controller.cpp

@@ -949,7 +949,7 @@ void play_controller::check_victory()
 	}
 
 	if (found_player || found_network_player) {
-		pump().fire("enemies defeated");
+		pump().fire("enemies_defeated");
 		if (is_regular_game_end()) {
 			return;
 		}