Sort: Change lexsort comment from stable to unstable (#8245)

# Which issue does this PR close?

The doc for lexsort says it's stable. However, it's an unstable sort.

# Rationale for this change

Fix the document.

# What changes are included in this PR?

Fix the document.

# Are these changes tested?

No need

# Are there any user-facing changes?

Doc change

---------

Co-authored-by: Matthijs Brobbel <m1brobbel@gmail.com>

Author: mapleFU

arrow-ord/src/sort.rs

@@ -841,7 +841,7 @@ pub struct SortColumn {
 
 /// Sort a list of `ArrayRef` using `SortOptions` provided for each array.
 ///
-/// Performs a stable lexicographical sort on values and indices.
+/// Performs an unstable lexicographical sort on values and indices.
 ///
 /// Returns an `ArrowError::ComputeError(String)` if any of the array type is either unsupported by
 /// `lexsort_to_indices` or `take`.