arrow

MINOR: [Docs] Fix nNull typo in pyarrow.compute.sort_indices docstring (#43251)



### Rationale for this change

Minor typo fix for the [sort_indices docstring](https://arrow.apache.org/docs/python/generated/pyarrow.compute.sort_indices.html)

### What changes are included in this PR?

`"nNull"` typo to `"null"`

### Are these changes tested?

Untested, just a single char change.

### Are there any user-facing changes?

Yes affects https://arrow.apache.org/docs/python/generated/pyarrow.compute.sort_indices.html

Authored-by: Alex Malins <github@alexmalins.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>