Fix IntervalIndex.symmetric_difference typo

Author: jschendel

doc/source/whatsnew/v0.21.1.txt

@@ -40,7 +40,7 @@ Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Improved performance of plotting large series/dataframes (:issue:`18236`).
--
+- Improved performance of ``IntervalIndex.symmetric_difference()`` (:issue:`18475`)
 -
 
 .. _whatsnew_0211.docs:
@@ -73,7 +73,7 @@ Indexing
 - Bug in a boolean comparison of a ``datetime.datetime`` and a ``datetime64[ns]`` dtype Series (:issue:`17965`)
 - Bug where a ``MultiIndex`` with more than a million records was not raising ``AttributeError`` when trying to access a missing attribute (:issue:`18165`)
 - Bug in :class:`IntervalIndex` constructor when a list of intervals is passed with non-default ``closed`` (:issue:`18334`)
--
+- Bug in ``IntervalIndex.symmetric_difference()`` where the symmetric difference with a non-``IntervalIndex`` did not raise (:issue:`18475`)
 -
 
 I/O

pandas/core/indexes/interval.py

@@ -1164,7 +1164,7 @@ def func(self, other):
     union = _setop('union')
     intersection = _setop('intersection')
     difference = _setop('difference')
-    symmetric_differnce = _setop('symmetric_difference')
+    symmetric_difference = _setop('symmetric_difference')
 
     # TODO: arithmetic operations