DOC: correct Panel.apply exsample (pandas-dev#19766)

Author: ZhuBaohe

pandas/core/panel.py

@@ -1020,7 +1020,7 @@ def apply(self, func, axis='major', **kwargs):
 
         Equivalent to previous:
 
-        >>> p.apply(lambda x: x.sum(), axis='minor')
+        >>> p.apply(lambda x: x.sum(), axis='major')
 
         Return the shapes of each DataFrame over axis 2 (i.e the shapes of
         items x major), as a Series