fixed help message about history cache size

IPython/core/interactiveshell.py

@@ -237,8 +237,8 b' class InteractiveShell(SingletonConfigurable):'
         """
         Set the size of the output cache.  The default is 1000, you can
         change it permanently in your config file.  Setting it to 0 completely
-        disables the caching system, and the minimum value accepted is 20 (if
-        you provide a value less than 20, it is reset to 0 and a warning is
+        disables the caching system, and the minimum value accepted is 3 (if
+        you provide a value less than 3, it is reset to 0 and a warning is
         issued).  This limit is defined because otherwise you'll spend more
         time re-flushing a too small cache than working
         """