Fix typo in the docstring of dropna

It should be 'default' instead of 'defalt'.

Author: berendt

pandas/core/frame.py

@@ -2933,7 +2933,7 @@ def dropna(self, axis=0, how='any', thresh=None, subset=None,
         subset : array-like
             Labels along other axis to consider, e.g. if you are dropping rows
             these would be a list of columns to include
-        inplace : boolean, defalt False
+        inplace : boolean, default False
             If True, do operation inplace and return None.
 
         Returns