[Minor] Adjust metric score when changing symbol's score

src/lua/lua_task.c

 	const gchar *symbol_name, *param;
 	struct rspamd_metric_result *metric_res;
 	struct rspamd_symbol_result *s = NULL;
-	struct rspamd_symbol_option *opt;
 	double weight;
 	gint i, top;
 
 		}
 
 		if (s) {
+			metric_res->score -= s->score;
 			s->score = weight;
+			metric_res->score += s->score;
 		}
 		else {
 			return luaL_error (L, "symbol not found: %s", symbol_name);