Display the correct error message when calling a binary moment without appropriate parameters.

Author: tr11

pandas/stats/moments.py

@@ -195,7 +195,7 @@ def _get_corr(a, b):
 
 def _flex_binary_moment(arg1, arg2, f):
     if not (isinstance(arg1,(np.ndarray, Series, DataFrame)) and
-            isinstance(arg1,(np.ndarray, Series, DataFrame))):
+            isinstance(arg2,(np.ndarray, Series, DataFrame))):
         raise ValueError("arguments to moment function must be of type ndarray/DataFrame")
 
     if isinstance(arg1, (np.ndarray,Series)) and isinstance(arg2, (np.ndarray,Series)):