Apply suggestions from code review

Co-authored-by: Weston Pace <[email protected]>

python/pyarrow/_compute.pyx

@@ -2802,7 +2802,7 @@ def register_vector_function(func, function_name, function_doc, in_types, out_ty
     This API is EXPERIMENTAL.
 
     A vector function is a function that executes vector
-    operations on arrays. Unlike scalar function, vector
+    operations on arrays. Unlike scalar functions, a vector
     function often has a grouping semantics and the output
     for a row depends on other rows. A typical example
     of vector function is "rank".