Clarify error message

Author: mgbckr

memory_profiler.py

@@ -152,7 +152,7 @@ def _ps_util_full_tool(memory_metric):
         process = psutil.Process(pid)
         try:
             if not hasattr(process, 'memory_full_info'):
-                raise NotImplementedError("Backend `ps_util_pss` requires psutil > 4.0.0")
+                raise NotImplementedError("Backend `psutil_pss` and `psutil_uss` requires psutil > 4.0.0")
 
             meminfo_attr = 'memory_full_info'
             meminfo = getattr(process, meminfo_attr)()