ooops, fix error message in execusercustomize()

Copy/paste failure :-)
diff --git a/Lib/site.py b/Lib/site.py
index 02129aa..e391670 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -508,7 +508,7 @@
             sys.excepthook(*sys.exc_info())
         else:
             print>>sys.stderr, \
-                "'import sitecustomize' failed; use -v for traceback"
+                "'import usercustomize' failed; use -v for traceback"
 
 
 def main():