typo fixed

observation_sim/sim_steps/add_objects.py

@@ -245,7 +245,7 @@ def add_objects(self, chip, filt, tel, pointing, catalog, obs_param):
         obj.unload_SED()
         del obj
         # gc.collect()
-    cat.free_mem
+    cat.free_mem()
     del cat
 
     if chip.survey_type == "spectroscopic" and not self.overall_config["run_option"]["out_cat_only"] and chip.slsPSFOptim: