fix misspelled word (#15097)

`colunwise` should be `columnwise`

Author: uhho

pandas/formats/style.py

@@ -781,7 +781,7 @@ def background_gradient(self, cmap='PuBu', low=0, high=0, axis=0,
         low, high: float
             compress the range by these values.
         axis: int or str
-            1 or 'columns' for colunwise, 0 or 'index' for rowwise
+            1 or 'columns' for columnwise, 0 or 'index' for rowwise
         subset: IndexSlice
             a valid slice for ``data`` to limit the style application to