Correct typo in warning message.

Change `# duplicate columns & possible reduce dimensionaility` to `# duplicate columns & possible reduce dimensionality`.

Author: Brian J. McGuirk

pandas/core/frame.py

@@ -1925,7 +1925,7 @@ def _getitem_column(self, key):
         if self.columns.is_unique:
             return self._get_item_cache(key)
 
-        # duplicate columns & possible reduce dimensionaility
+        # duplicate columns & possible reduce dimensionality
         result = self._constructor(self._data.get(key))
         if result.columns.is_unique:
             result = result[key]